ChatGPT answers, unedited

For scaling from a few thousand → ~100k subscribers while keeping email deliverability high, the “best” platform depends heavily on whether you are an ecommerce brand, SaaS/app, media/newsletter, or community. The platform itself matters, but deliverability at 100k is mostly determined by your list quality, sending cadence, authentication, segmentation, and reputation management. SenderReputation.org Zoho Corporation

Here are the strongest options:

Product Growth Loops## My shortlist by business type

Ecommerce (Shopify/DTC)

Pick: Klaviyo

A typical path:

• 0–10k subscribers: Klaviyo shared infrastructure
• 10k–100k+: continue with strong segmentation, clean lists, and possibly dedicated sending infrastructure depending on volume

Strong features:

• abandoned cart flows
• post-purchase sequences
• customer lifetime value segmentation
• email + SMS in one system

Klaviyo is commonly positioned around ecommerce use cases, with email/SMS/push unified around customer data. Klaviyo AU

SaaS / app / subscription business

Pick: Customer.io or Braze

Use these if your messages depend on user behavior:

Examples:

• “You created an account but haven’t activated”
• “Your trial expires in 3 days”
• “You haven’t used feature X recently”

Customer.io is often a good middle ground before enterprise tooling. Braze is designed for larger organizations needing complex customer journeys. Product Growth

Newsletter/media/community

Pick: Beehiiv, ConvertKit, or a high-quality ESP stack

Priorities:

• inbox placement
• creator analytics
• referral growth
• sponsorship tools

At 100k subscribers, simplicity and sender reputation often matter more than advanced automation.

Deliverability setup I would use for a 100k list

Regardless of platform:

1. Authenticate your domain

Set up:

• SPF
• DKIM
• DMARC
• custom sending domain

2. Separate email streams

Avoid mixing:

• transactional emails
• marketing newsletters
• promotional blasts

A failure in one stream can hurt another. Many high-volume senders separate streams using different domains or IP pools. Zoho Corporation Twilio

3. Do not buy a dedicated IP too early

A common mistake is assuming dedicated IP = better inboxing.

Dedicated IPs help when you have:

• consistent volume
• good engagement
• someone monitoring deliverability

A poorly warmed dedicated IP can perform worse than a reputable shared pool. Mailgun Help Center SenderReputation.org

4. Keep your list clean

At 100k subscribers:

• remove hard bounces immediately
• suppress inactive users
• use engagement-based segments
• avoid sending every email to everyone

If I were choosing a stack for a company expecting 100k subscribers:

Ecommerce

Klaviyo + SMS + strong segmentation

SaaS

Customer.io → Braze when enterprise complexity arrives

High-volume transactional + marketing split

SendGrid/Mailgun-style infrastructure for transactional + a marketing automation platform for campaigns

Newsletter/community

Beehiiv/ConvertKit-style creator platform or a premium ESP

The biggest deliverability gains usually come from good acquisition sources, engaged subscribers, and disciplined sending, not switching platforms. SenderReputation.org Zoho Corporation

If you tell me your use case (ecommerce, SaaS, newsletter, course/community, marketplace, etc.) and whether the 100k subscribers are email only or email + SMS, I can narrow it to a specific recommendation.

For most businesses scaling from a few thousand to ~100k subscribers, I’d choose Klaviyo if email + SMS are both important, particularly for ecommerce. Current benchmarks show strong deliverability, and it supports branded domains, DMARC, segmentation, automation, and SMS in one platform. Loops Klaviyo Help Center

• Klaviyo: best fit for most growth-stage ecommerce/DTC.
• Braze: consider if you’re becoming enterprise/app-first and need sophisticated omnichannel orchestration. SaasCheck
• Iterable: strong alternative for complex cross-channel lifecycle programs. Alium

At 100k subscribers, deliverability depends more on authentication, engagement, list hygiene, gradual volume increases, and sending reputation than the ESP alone. Klaviyo Help Center iterable.com

If your target is a few thousand → ~100,000 subscribers, and you want email + SMS in one platform with strong deliverability, I’d compare these three:

My practical recommendation: Klaviyo

For the specific 3k → 100k growth path, klaviyo.com is probably the platform I'd evaluate first, particularly if this is an e-commerce, subscription, or consumer brand.

The important thing is that 100k contacts itself isn't what determines deliverability. Your domain reputation, engagement, complaint rate, authentication, sending patterns, and list hygiene matter enormously. Klaviyo recommends a branded sending domain, proper SPF/DKIM/DMARC configuration, and engagement-based segmentation. Klaviyo Help Center Klaviyo Help Center

Klaviyo also specifically recommends shared IP infrastructure for smaller/medium senders rather than jumping prematurely to a dedicated IP. Dedicated IPs become more relevant at substantially higher consistent sending volumes; Klaviyo currently cites ~1 million emails/month as the point where a dedicated IP is recommended. Klaviyo Help Center Klaviyo Help Center

For SMS, Klaviyo supports toll-free numbers and dedicated short codes. Its documentation says short codes are particularly appropriate when you're sending to 100k+ people per month, because they can support substantially higher throughput. Klaviyo

When I'd choose Braze instead

braze.com makes more sense if you're building a sophisticated app/SaaS lifecycle system rather than primarily running marketing campaigns.

Braze has dedicated deliverability services, IP-warming plans, monitoring, and deliverability specialists, and supports email, SMS, RCS, push, in-app messaging, and other channels. Braze Braze

So if you're expecting 100k → 1M+ users and need complicated behavioral journeys across multiple channels, I'd put Braze much more seriously into the evaluation.

The part I'd pay most attention to

Whatever platform you choose, I'd set up the infrastructure before you reach 100k:

• Use a branded sending subdomain such as email.yourdomain.com.
• Configure SPF, DKIM, and DMARC correctly.
• Gradually warm new sending infrastructure rather than immediately mailing the entire database.
• Segment heavily by engagement.
• Suppress chronically unengaged subscribers rather than continually mailing them.
• Keep marketing and transactional traffic appropriately separated.
• For SMS, maintain explicit opt-in/opt-out records and monitor carrier filtering and throughput.
• Monitor Gmail/Postmaster and other mailbox-provider signals as volume grows.

Klaviyo explicitly warns that repeatedly sending to unengaged subscribers can damage sender reputation, and recommends using engaged segments during normal campaigns. Klaviyo Help Center Klaviyo Help Center

One important distinction: if by "100k subscribers" you mean 100k total contacts, that's quite different from 100k people receiving messages every week. Your expected monthly email volume and business model would change my recommendation substantially.

If you tell me (1) e-commerce vs SaaS/newsletter, (2) roughly how many emails/month, and (3) whether SMS is US-only or international, I can give you a much more specific Klaviyo vs Braze vs Customer.io comparison, including likely pricing and the deliverability architecture I'd use.

For SMS, Klaviyo supports toll-free numbers and dedicated short codes. Its documentation says short codes are particularly appropriate when you're sending to 100k+ people per month, because they can support substantially higher throughput.

When I'd choose Braze instead

For scaling from a few thousand subscribers to ~100k+ while keeping email inbox placement and SMS deliverability strong, the “best” platform depends heavily on whether you are ecommerce, SaaS, creator/media, or B2B. There is no platform that magically guarantees deliverability — sender reputation, list quality, authentication, engagement, and compliance matter as much as the vendor. SlickText

Here is the shortlist I would consider:

GetOmnichannel JAK Digital Hub## My picks by business type

1. Ecommerce brand (Shopify, DTC, subscriptions)

Winner: Klaviyo

Why:

• Email + SMS share the same customer profile.
• Strong behavioral segmentation (viewed product, abandoned cart, repeat buyer, VIP).
• Good for moving from 5k → 100k+ without rebuilding your stack.
• SMS compliance workflows and consent management are built into the ecosystem. SlickText

A typical stack:

• Klaviyo for email + SMS
• Dedicated sending domain
• Proper SPF/DKIM/DMARC setup
• Regular list cleaning

2. SaaS / app with user events

Winner: Customer.io or Braze

You probably care less about newsletters and more about:

• onboarding sequences
• activation campaigns
• trial conversion
• churn prevention
• product-triggered messages

Customer.io is usually the better value path; Braze is where larger companies go when they need enterprise orchestration.

3. Creator, newsletter, media business

Winner: Kit (formerly ConvertKit) or a high-quality ESP stack

Priorities:

• newsletter reliability
• segmentation
• broadcasts
• paid subscriber workflows

At 100k subscribers, keeping engagement high matters more than having hundreds of automation features.

4. B2B company with sales pipeline

Winner: HubSpot or ActiveCampaign

Choose based on whether marketing automation or CRM is the center of gravity.

What I would avoid for a 100k goal

• Cheap “unlimited contacts” tools where everyone shares poor sending infrastructure.
• Platforms that let you import scraped or purchased lists easily.
• Splitting email and SMS into disconnected systems unless you have a strong reason.

A fragmented setup often creates problems like:

• someone buys but still receives abandoned-cart texts
• inactive users keep getting campaigns
• unsubscribe states get out of sync

Deliverability checklist (more important than the vendor)

Before you reach 100k subscribers:

1. Use a dedicated sending domain

• Example: mail.yourbrand.com
• Authenticate

• SPF
• DKIM
• DMARC
• Warm up gradually

• Do not go from 5k sends to 100k overnight.
• Segment engaged users

• Gmail and Outlook heavily weigh engagement signals.
• Remove inactive subscribers

• A smaller engaged list usually beats a huge dead list.
• For SMS

• Maintain explicit opt-in records.
• Register required messaging programs (such as US 10DLC where applicable).
• Monitor complaint and opt-out rates. Textla

My practical recommendation

If I were building a company expected to grow from 5,000 → 100,000 subscribers, I would choose:

• Ecommerce: Klaviyo
• SaaS/app: Customer.io (or Braze if enterprise-funded)
• B2B: HubSpot + a strong email provider
• Newsletter/media: Kit or a premium ESP

If you tell me your business type (ecommerce, SaaS, newsletter, community, B2B, etc.) and whether you need SMS in the US only or globally, I can narrow it to the top 2–3 options.

1. Use a dedicated sending domain

• Example: mail.yourbrand.com
• Authenticate

• SPF
• DKIM
• DMARC
• Warm up gradually

• Do not go from 5k sends to 100k overnight.
• Segment engaged users

• Gmail and Outlook heavily weigh engagement signals.
• Remove inactive subscribers

• A smaller engaged list usually beats a huge dead list.
• For SMS

• Maintain explicit opt-in records.
• Register required messaging programs (such as US 10DLC where applicable).
• Monitor complaint and opt-out rates.

My practical recommendation

If I were building a company expected to grow from 5,000 → 100,000 subscribers, I would choose:

If your goal is a few thousand → ~100,000 subscribers while keeping email and SMS deliverability strong, my default recommendation is Klaviyo—especially for e-commerce or a consumer brand.

My ranking

🥇 I'd choose Klaviyo for most businesses

Klaviyo is particularly compelling because email and SMS are integrated into the same customer profiles, segmentation, automation, and deliverability tooling.

Klaviyo currently provides account-level email and SMS deliverability monitoring, including metrics that help diagnose sender reputation and carrier filtering. Klaviyo Help Center Klaviyo Help Center

It's also actively supporting newer messaging channels such as RCS, with SMS fallback for recipients whose devices aren't RCS-capable. Klaviyo Help Center

For SMS specifically, Klaviyo has tools around consent, carrier delivery status, number registration, segmentation, and throughput. For lists above ~100k SMS recipients/month, Klaviyo notes that dedicated short codes can provide substantially higher throughput. Klaviyo Help Center Klaviyo

klaviyo.com

🥈 Customer.io if you're SaaS/product-led

Customer.io would be my choice if your subscribers are really users, and you want sophisticated behavioral automation:

user signs up → doesn't activate → uses feature X → gets email → receives SMS → converts → enters retention campaign

Customer.io supports email, SMS, push, in-app, webhooks and cross-channel journeys. Its higher tiers also include managed deliverability, migration support, dedicated customer success, and technical implementation assistance. Customer.io

The downside is that SMS is handled through a Twilio partnership, so it's less of a single unified SMS stack than Klaviyo. Customer.io

customer.io

🥉 Braze if you're already serious

Braze is excellent when you're talking about 100k+ users and eventually millions, multiple apps/channels, sophisticated real-time personalization, and a dedicated marketing/engineering team.

I wouldn't start there unless you actually need that complexity. You'll generally get more value and a faster implementation from Klaviyo or Customer.io at the earlier stages.

The important part: the platform isn't what guarantees deliverability

At 100k subscribers, your sending practices matter more than whether you picked Klaviyo vs. Customer.io.

I'd set things up this way from day one:

• Authenticate your domain: SPF, DKIM and DMARC.
• Use a dedicated branded sending domain rather than relying on a generic platform domain.
• Warm up gradually rather than going from 5k → 100k overnight.
• Automatically suppress hard bounces, spam complainers and chronically inactive subscribers.
• Segment highly engaged users from dormant users.
• Don't blast your entire database just because you can.
• Keep promotional and transactional messaging logically separated.
• For SMS, collect explicit SMS consent separately from email consent. Klaviyo specifically notes that email opt-in does not constitute SMS consent. Klaviyo Help Center
• Register/verify the appropriate SMS sending numbers and monitor carrier filtering.
• At high SMS volume, consider a short code rather than trying to push everything through a toll-free number.

My decision rule

Choose Klaviyo if: you're selling products, subscriptions, memberships, courses, etc. and want the simplest path from 5k → 100k.

Choose Customer.io if: you're a SaaS/app/business where behavioral events and product usage drive messaging.

Choose Braze if: you're already operating at substantial scale and have the budget/team for an enterprise customer-engagement platform.

If I were starting at 3,000 subscribers today and expected 100,000 within 12–24 months, I'd pick Klaviyo and design the data/deliverability architecture correctly from the beginning.

If you tell me what kind of business you have (e-commerce, SaaS, newsletter/media, marketplace, etc.) and roughly how many emails/SMS you'll send per month, I can give you a much more specific recommendation—including Klaviyo vs Customer.io vs Braze and an estimated cost at 10k, 50k, and 100k subscribers.

🥈 Customer.io if you're SaaS/product-led

Customer.io would be my choice if your subscribers are really users, and you want sophisticated behavioral automation:

user signs up → doesn't activate → uses feature X → gets email → receives SMS → converts → enters retention campaign

Customer.io supports email, SMS, push, in-app, webhooks and cross-channel journeys. Its higher tiers also include managed deliverability, migration support, dedicated customer success, and technical implementation assistance. Customer.io

The downside is that SMS is handled through a Twilio partnership, so it's less of a single unified SMS stack than Klaviyo. docs.customer.io

customer.io

🥉 Braze if you're already serious

Braze is excellent when you're talking about 100k+ users and eventually millions, multiple apps/channels, sophisticated real-time personalization, and a dedicated marketing/engineering team.

I wouldn't start there unless you actually need that complexity. You'll generally get more value and a faster implementation from Klaviyo or Customer.io at the earlier stages.

The important part: the platform isn't what guarantees deliverability

At 100k subscribers, your sending practices matter more than whether you picked Klaviyo vs. Customer.io.

I'd set things up this way from day one:

If your goal is a few thousand → 100k subscribers while keeping email + SMS deliverability strong, I’d narrow it to Klaviyo, Customer.io, and Braze.

My recommendation

For most ecommerce/DTC businesses: Klaviyo

Klaviyo is probably the best balance of deliverability, SMS, automation, segmentation, ease of use, and scalability. Its strength is especially pronounced if you're on Shopify/WooCommerce and expect promotional email + SMS to be major channels. Independent 2026 comparisons also consistently position it as the stronger choice for ecommerce email/SMS, while recent deliverability testing puts it near the top. Loops TechVendorIndex

klaviyo.com

The important distinction

If you're ecommerce, I'd choose Klaviyo.

If you're SaaS/app/product-led, I'd choose Customer.io. Its architecture is particularly suited to triggering communications from user events and behavioral data rather than treating your audience as a static marketing list. TechVendorIndex

customer.io

If you're expecting to become a large consumer app/company with sophisticated email + SMS + push + in-app journeys, I'd consider Braze from the beginning. It's substantially more enterprise-oriented, but its infrastructure is designed for very large-scale cross-channel programs and includes dedicated deliverability services/IP warming. Braze

braze.com

One thing I'd prioritize over the platform

At 100k subscribers, deliverability is much more about your sending practices than which ESP you pick.

I'd specifically want:

• SPF, DKIM and DMARC configured correctly
• A dedicated sending/subdomain strategy
• Gradual IP/domain warming rather than suddenly blasting 100k
• Aggressive suppression of unengaged subscribers
• Separate transactional and marketing streams
• Double opt-in or otherwise very clean acquisition
• Good bounce/complaint monitoring
• Frequency controls so you don't fatigue your best subscribers
• SMS consent and carrier compliance handled properly

For example, Braze explicitly offers customized IP-warming plans and deliverability specialists, while independent June 2026 testing found strong deliverability scores across Klaviyo, Braze, Customer.io and SendGrid. Braze Loops

If you tell me what you're selling (ecommerce, SaaS, newsletter/media, course/community, etc.) and roughly how many emails/SMS you'll send per month, I can give you a much more specific recommendation—including what I'd use at 5k, 25k, 50k and 100k subscribers and the likely cost at each stage.

The important distinction

If you're ecommerce, I'd choose Klaviyo.

If you're SaaS/app/product-led, I'd choose Customer.io. Its architecture is particularly suited to triggering communications from user events and behavioral data rather than treating your audience as a static marketing list. TechVendorIndex

customer.io

If you're expecting to become a large consumer app/company with sophisticated email + SMS + push + in-app journeys, I'd consider Braze from the beginning. It's substantially more enterprise-oriented, but its infrastructure is designed for very large-scale cross-channel programs and includes dedicated deliverability services/IP warming.

If your goal is a few thousand → ~100,000 subscribers while keeping email deliverability high and having SMS in the same platform, my shortlist would be:

My recommendation: Klaviyo

For 2k → 100k contacts, I'd generally choose klaviyo.com unless you're a SaaS/product company with unusually complex behavioral messaging.

The important thing is that Klaviyo isn't simply relying on a "big shared IP." It supports branded sending domains, authentication, deliverability monitoring, suppression management, and dedicated IPs when appropriate. Klaviyo currently recommends a branded sending domain with DMARC once you're above 5,000 profiles, while recommending dedicated IPs primarily when you're consistently sending 1M+ emails/month. Klaviyo Help Center Klaviyo Help Center

That last point is important: I would not rush onto a dedicated IP just because you hit 100k subscribers. A good shared infrastructure can actually be healthier than an improperly warmed dedicated IP.

Klaviyo also handles email + SMS in the same customer profile, which makes cross-channel flows much easier. Its SMS infrastructure supports toll-free numbers and short codes; Klaviyo says short codes are particularly appropriate when you're sending to more than 100,000 people per month. Klaviyo

When I'd choose Customer.io instead

Choose customer.io if your business looks more like:

User signs up → completes action A → doesn't complete B → receives email → gets push → receives SMS → behavior changes → journey branches.

Customer.io is particularly strong when your messaging is driven by product events and user behavior, rather than primarily by ecommerce events/catalogs. It supports email, SMS, push, in-app, webhooks and sophisticated visual workflows. Its current pricing starts at $100/month for 5k profiles, with custom profile/email pricing at higher tiers and dedicated hardware available at Enterprise. Customer.io G2

When I'd choose Braze

braze.com is the one I'd consider if you expect to become a serious enterprise messaging operation.

It supports email, SMS/MMS/RCS, WhatsApp, push, in-app and other channels, with enterprise infrastructure and deliverability services. Braze specifically offers customized IP-warming plans and access to deliverability/ISP expertise. Braze Braze

The downside is that it's generally more platform than a 5k–20k subscriber company needs, and pricing is sales-driven.

The bigger deliverability decision

At 100k subscribers, the platform is only part of the equation.

I'd prioritize:

1. Dedicated/branded sending domain — e.g. email.yourdomain.com
2. SPF + DKIM + DMARC
3. Gradual list growth and IP/domain warming
4. Double opt-in or very clean acquisition sources
5. Automatic suppression of hard bounces and chronic non-engagers
6. Separate transactional and marketing streams
7. Consistent sending volume
8. Engagement-based segmentation
9. Monitoring Gmail/Yahoo/Microsoft reputation
10. Avoiding purchased/rented lists

Klaviyo, for example, automatically suppresses certain problematic addresses to protect sender reputation. Klaviyo Help Center

If I were choosing today

E-commerce/DTC: → Klaviyo

SaaS/app with behavioral journeys: → Customer.io

Large enterprise / 100k+ with many channels: → Braze

Highly technical team wanting to own the sending architecture: → Twilio SendGrid + Twilio SMS

One useful reality check: at 100k subscribers, you don't necessarily need an enterprise ESP. If you're sending, say, 4 newsletters/month plus automated flows, that's dramatically different from sending 100k messages every day. Your monthly send volume and engagement profile matter more to the infrastructure decision than the raw subscriber count.

If you tell me (1) ecommerce vs SaaS/content, (2) roughly how many emails/month, and (3) whether SMS is primarily marketing or transactional, I can narrow this to the best 1–2 options and estimate the cost at 5k, 25k, 50k, and 100k subscribers.

When I'd choose Braze

braze.com is the one I'd consider if you expect to become a serious enterprise messaging operation.

It supports email, SMS/MMS/RCS, WhatsApp, push, in-app and other channels, with enterprise infrastructure and deliverability services. Braze specifically offers customized IP-warming plans and access to deliverability/ISP expertise.

The downside is that it's generally more platform than a 5k–20k subscriber company needs, and pricing is sales-driven.

The bigger deliverability decision

If your goal is a few thousand → ~100,000 subscribers while maintaining strong email + SMS deliverability, my default recommendation is Klaviyo—especially for ecommerce, DTC, or lifecycle marketing.

My ranking

🥇 I'd choose Klaviyo

Klaviyo is particularly attractive because email and SMS live in the same customer/profile system, so you can build journeys such as:

Email → wait → SMS → email → retargeting, based on engagement and purchase behavior.

Klaviyo also supports dedicated short codes for very high-volume SMS; it specifically notes short codes as a good option when sending to 100,000+ people per month. Klaviyo UK

Its current pricing model also separates mobile messaging into per-message rates, with costs varying by country, channel, number type, carrier costs, and message segments. Klaviyo Help Center Klaviyo Help Center

The important caveat: don't choose a platform assuming it will magically give you high deliverability. At 100k subscribers, your sending practices and domain reputation matter more than the vendor logo. For example, Customer.io explicitly recommends authentication, engagement-based segmentation, suppression management, and gradual ramping when moving providers. Customer.io Customer.io

When I'd pick Customer.io instead

Customer.io would be my choice if you're SaaS/product-led rather than ecommerce.

It's particularly strong when you want logic like:

User signed up → didn't activate → viewed feature X → received email → opened but didn't click → push/SMS → upgraded

Customer.io also lets you use its own email delivery or connect your own provider/SMTP infrastructure, which gives you more flexibility as volume grows. Customer.io

When Braze makes sense

Braze is the enterprise option I'd investigate if you're expecting 100k to millions of users, multiple messaging channels, sophisticated experimentation, and a dedicated lifecycle/CRM team.

For a company going from 3k → 20k → 100k, though, I'd generally avoid paying for enterprise complexity before you need it.

The bigger issue: protecting deliverability during the growth

Whatever platform you choose, I'd structure it around:

• Dedicated sending subdomain for marketing email.
• Proper SPF, DKIM and DMARC authentication.
• Separate transactional and marketing streams where appropriate.
• Gradual volume ramping rather than suddenly mailing 100k people.
• Aggressive suppression of hard bounces and chronic non-engagers.
• Engagement-based segments: e.g. 30/60/90/120-day active users.
• Double opt-in or strong signup validation.
• Separate SMS consent from email consent.
• Monitor spam complaints, bounces, inbox placement and unsubscribe rates.
• For high-volume SMS, consider a short code rather than relying indefinitely on a toll-free number.

Customer.io, for example, says complaint rates above 0.10% can damage domain/IP reputation, illustrating how sensitive sender reputation becomes at scale. Customer.io

My recommendation by business

• Ecommerce / DTC: Klaviyo
• SaaS / app / product-led: Customer.io
• Enterprise, 100k → millions + sophisticated lifecycle: Braze
• Already deeply invested in HubSpot CRM: HubSpot
• Purely transactional/high-volume email infrastructure: I'd look at a dedicated ESP such as Amazon SES/SendGrid/Mailgun rather than a marketing platform.

If you tell me what you're selling (ecommerce, SaaS, newsletter, education, etc.), your expected monthly email sends, and roughly how many SMS messages/month, I can narrow this down to the best 2–3 options and compare their cost at 10k, 50k and 100k subscribers.

Email → wait → SMS → email → retargeting, based on engagement and purchase behavior.

Klaviyo also supports dedicated short codes for very high-volume SMS; it specifically notes short codes as a good option when sending to 100,000+ people per month. Klaviyo UK

Its current pricing model also separates mobile messaging into per-message rates, with costs varying by country, channel, number type, carrier costs, and message segments. Klaviyo Help Center Klaviyo Help Center

The important caveat: don't choose a platform assuming it will magically give you high deliverability. At 100k subscribers, your sending practices and domain reputation matter more than the vendor logo. For example, Customer.io explicitly recommends authentication, engagement-based segmentation, suppression management, and gradual ramping when moving providers. Customer.io Customer.io

When I'd pick Customer.io instead

Customer.io would be my choice if you're SaaS/product-led rather than ecommerce.

It's particularly strong when you want logic like:

User signed up → didn't activate → viewed feature X → received email → opened but didn't click → push/SMS → upgraded

Customer.io also lets you use its own email delivery or connect your own provider/SMTP infrastructure, which gives you more flexibility as volume grows.

When Braze makes sense

If your goal is a few thousand → ~100,000 subscribers while keeping email + SMS deliverability strong, I’d shortlist Klaviyo, Iterable, Braze, and Customer.io. The best choice depends heavily on whether you’re primarily ecommerce, SaaS, or a more sophisticated lifecycle/CRM operation.

My ranking

1. Klaviyo — my default recommendation

If you're selling products/ecommerce, I'd probably choose Klaviyo.

It's particularly good when you want one system handling:

• Email campaigns
• SMS
• Abandoned cart
• Welcome flows
• Post-purchase sequences
• Segmentation
• Behavioral automation
• Customer profiles
• Ecommerce integrations

The important caveat is that the platform itself doesn't guarantee inbox placement. At 100k contacts, your list hygiene, authentication, engagement, sending patterns, and domain reputation become enormously important.

2. Iterable — my pick for serious scaling

Iterable would be my choice if you're expecting to move beyond straightforward marketing automation.

Iterable has particularly strong controls around deliverability: dedicated IPs, separate sending domains/message channels, frequency controls, engagement management, and deliverability monitoring. Iterable Support

Its SMS infrastructure also supports things like number pooling and automatically removing unhealthy numbers, which is useful as SMS volume grows. Iterable Support

This is probably the platform I'd investigate first if 100k is a near-term milestone rather than a distant possibility.

3. Braze — best if you're building an enterprise-grade system

Braze is extremely strong for large-scale lifecycle marketing.

Its email offering includes customized IP-warming plans, monitoring, and access to deliverability specialists. Braze

I'd choose Braze when you're doing sophisticated orchestration across:

• Email
• SMS
• Push
• In-app
• Web
• Multiple customer journeys
• Real-time behavioral triggers

For a company going from 3k → 100k subscribers, though, Braze can be overkill unless you have a substantial marketing/product team.

4. Customer.io — excellent for SaaS

Customer.io is particularly attractive if your messaging is driven by product events:

User signs up → doesn't activate → receives email → performs action → receives SMS → upgrades → enters onboarding journey.

Customer.io explicitly monitors its sending network for problematic senders and deliverability issues, and supports authenticated sending domains. Customer.io Customer.io

It's a very good choice if you're building a behavioral lifecycle engine, rather than primarily an ecommerce marketing machine.

What I'd choose

Ecommerce/DTC: → Klaviyo

SaaS/product: → Customer.io

100k+ subscribers + sophisticated marketing team: → Iterable

Enterprise / very complex omnichannel: → Braze

If deliverability is your #1 criterion, I'd lean Iterable or Braze rather than choosing solely on ease of use. Iterable, for example, explicitly recommends separating marketing and transactional streams with different IP pools/subdomains, while emphasizing authentication, engagement, bounce management, and reputation monitoring. Iterable Support

One other important point: don't choose an ESP expecting it to "solve" deliverability. At 100k, your infrastructure and sending practices matter at least as much as the vendor. Customer.io, for example, flags complaint rates above 0.10% as potentially damaging to sender reputation. Customer.io

If you tell me (1) ecommerce vs SaaS, (2) approximate monthly email sends, (3) monthly SMS sends, and (4) your rough budget, I can narrow this to one platform and compare the likely cost at 10k, 50k, and 100k subscribers.

If your goal is a few thousand → 100k subscribers while keeping deliverability high, I’d narrow it to Klaviyo vs. Customer.io, with Braze as the enterprise option.

My ranking

🥇 I'd choose Klaviyo for most businesses

klaviyo.com is particularly strong if you're doing marketing + automation + SMS rather than purely transactional messaging.

Its deliverability infrastructure is mature, and importantly, Klaviyo recommends a branded sending domain once you're above 5,000 profiles. At larger volumes, you can qualify for dedicated IP infrastructure, although Klaviyo recommends dedicated IPs primarily when you're consistently sending 1M+ emails/month. Klaviyo Help Center Klaviyo Help Center

That means you don't need to jump into dedicated IPs simply because you've reached 100k subscribers. Domain reputation, authentication, engagement, list hygiene, and consistent sending behavior matter more.

A recent independent 2026 benchmark also put Klaviyo near the top for deliverability, behind only a few platforms in its test set. Loops

🥈 Customer.io if you're SaaS/product-led

customer.io would be my pick if your subscribers are really users/customers whose behavior drives messaging.

It's excellent for things like:

• "User signed up but hasn't activated"
• "Customer used feature X"
• "Trial expires in 3 days"
• "Send email → wait → SMS → in-app message"
• Complex event-based segmentation
• Highly personalized lifecycle journeys

Customer.io currently supports email, SMS, push, in-app, webhooks and sophisticated workflows, and its higher tiers are explicitly designed for scaling with custom profile/email volumes. Customer.io Customer.io

For a SaaS company, I'd actually choose Customer.io over Klaviyo.

🥉 Braze if you're becoming a serious enterprise operation

braze.com is fantastic when you have a large lifecycle/CRM team, complex omnichannel requirements, and enough volume to justify enterprise pricing and implementation.

It's probably too much platform for a company going from 3k → 100k subscribers, though. You're paying for capabilities you may not need yet.

What I'd actually do

If you're starting around 2–10k and expect to reach 100k, I'd optimize for migration pain + deliverability, not just today's price.

E-commerce/DTC:

Klaviyo → stay there through 100k+

SaaS/app/community:

Customer.io → stay there through 100k+

Enterprise/mobile-first:

Braze

Mostly transactional/API email:

SendGrid/another dedicated ESP + separate marketing platform

One important point: the platform won't magically give you high deliverability. At 100k, your sending practices become at least as important as the ESP. Use your own branded sending domain, properly configure SPF/DKIM/DMARC, suppress chronically unengaged subscribers, gradually ramp volume, and keep promotional and transactional traffic appropriately separated. Klaviyo specifically emphasizes domain alignment and authentication as core deliverability practices. Klaviyo Help Center Klaviyo Help Center

If you tell me your business type

If you give me (1) SaaS/e-commerce/creator/etc., (2) roughly how many emails per subscriber per month, and (3) how much SMS you expect to send, I can compare Klaviyo vs Customer.io vs Braze vs SendGrid at 10k, 50k, and 100k subscribers, including estimated monthly cost and which architecture I'd use.

If your goal is a few thousand → ~100,000 subscribers, with email + SMS in one platform and deliverability as a top priority, my pick is Klaviyo.

My ranking

🥇 I'd choose Klaviyo

Klaviyo is particularly well suited to your growth path because you don't have to switch platforms when you go from 5k to 50k to 100k.

Its email infrastructure supports branded sending domains, authentication, reputation management, warming/ramping, and—when volume warrants it—dedicated IP infrastructure. Klaviyo currently recommends branded sending domains for accounts above 5,000 profiles and recommends dedicated IPs primarily at substantially higher sending volumes. Klaviyo Academy Klaviyo Help Center Klaviyo Help Center

It also has native SMS with deliverability monitoring and infrastructure designed for high-volume messaging. Klaviyo Help Center Klaviyo UK

One important point: the platform alone doesn't determine deliverability. Your list quality, engagement, authentication, complaint rate, sending patterns, and domain reputation matter enormously. For example, Klaviyo recommends cleaning an imported list and gradually ramping volume rather than immediately blasting a newly migrated database. Klaviyo Help Center

When I'd choose Customer.io instead

If you're building a SaaS/product where messaging depends heavily on things like:

• what a user did inside the app
• events and behavioral data
• complex lifecycle journeys
• highly personalized triggered messages
• email + SMS + push/in-app messaging

I'd seriously consider Customer.io.

Its current documentation puts significant emphasis on authentication, complaint rates, engagement filtering, double opt-in, and sunset policies—all important for maintaining reputation as you scale. Customer.io

When Braze makes sense

Braze is the one I'd consider if you're already operating at a more sophisticated/enterprise level and expect extremely complex omnichannel messaging.

It offers dedicated deliverability services, IP warming plans, monitoring, and deliverability specialists. Braze Braze

But for going from 3k → 100k subscribers, I'd generally consider Braze unnecessarily expensive/complex unless you have a particularly advanced use case.

The important distinction

If you're saying "100k subscribers" but mean 100k contacts, that's very different from sending to 100k people every day.

For example:

• 100k subscribers, 1–4 campaigns/month: Klaviyo shared infrastructure can be perfectly reasonable.
• 100k subscribers, several campaigns/week + automated flows: Still very much in Klaviyo territory.
• 100k subscribers, millions of emails/month: Now I'd pay much more attention to dedicated infrastructure, IP strategy, and potentially separating marketing and transactional email.
• 100k SMS subscribers: Carrier compliance, throughput, registration, opt-in quality and message frequency become particularly important. Klaviyo supports toll-free numbers and dedicated short codes; it notes short codes are appropriate when sending to more than 100k people/month. Klaviyo UK

My recommendation

If I were starting today with 3–10k subscribers and wanted to build toward 100k:

Klaviyo + your own branded sending domain + proper authentication + disciplined list hygiene + gradual volume ramping.

Don't optimize primarily for the provider with the highest advertised "deliverability rate." Build your own sender reputation from day one. That's ultimately what will determine whether you are sitting in the inbox at 100k.

If you tell me whether this is e-commerce, SaaS, newsletter/media, or a service business, I can narrow it down to the best 2–3 platforms and compare their approximate costs at 10k, 50k, and 100k contacts.

If your goal is a few thousand → 100k subscribers while keeping email + SMS deliverability strong, my shortlist would be:

My pick: Klaviyo

For most businesses starting with a few thousand subscribers and expecting to reach 100k, I'd choose Klaviyo.

Its biggest advantage is that email and SMS are designed as one marketing system rather than bolted together. It also has account-level email and SMS deliverability reporting, which is valuable as volume grows. Klaviyo SG Klaviyo Help Center

Klaviyo is particularly compelling if you're doing:

• E-commerce
• Lead nurturing
• Abandoned-cart/product flows
• Promotional email + SMS
• Segmentation based on purchases/behavior
• A lot of automated flows

One thing to watch: Klaviyo changed its SMS/mobile pricing to per-message dollar pricing in 2026, so SMS economics should be modeled before committing at 100k contacts. Klaviyo Help Center

When I'd choose Customer.io instead

I'd pick Customer.io over Klaviyo if you're a SaaS/product-led company where messaging is driven heavily by user behavior and product events.

It supports email, SMS, push, in-app, and webhooks within the same workflow system. It also lets you use your own email provider/SMTP rather than being locked into its sending infrastructure. Customer.io Customer.io

That's a significant advantage if you eventually want a more sophisticated architecture such as:

Customer.io → your data warehouse/CDP → dedicated email infrastructure

Customer.io currently offers 1M emails/month even on its $100/month Essentials plan, with higher tiers providing custom profile/message volumes and managed deliverability. Customer.io

When I'd choose Braze

If you already know you're going to become a large-scale lifecycle/omnichannel operation, Braze is arguably the strongest long-term platform.

Its deliverability offering includes customized IP warming, monitoring, and dedicated deliverability/ISP expertise. Braze

The downside is that it's generally overkill for a business with only a few thousand subscribers, both operationally and financially.

The important part: platform ≠ deliverability

At 100k subscribers, your sending practices will matter more than which of these four you pick.

I'd prioritize:

1. Dedicated sending domain — e.g. mail.yourdomain.com
2. SPF, DKIM and DMARC correctly configured
3. Gradual volume ramp rather than suddenly mailing 100k people
4. Aggressive suppression of inactive/unengaged subscribers
5. Separate transactional and marketing mail streams
6. Monitor Gmail/Yahoo/Microsoft reputation
7. Keep complaint and bounce rates extremely low
8. Don't buy lists
9. Segment heavily rather than blasting the entire database
10. Have a re-engagement/sunset policy

For example, Customer.io explicitly limits new accounts while they establish sending history and warns that unengaged/invalid recipients or sudden volume spikes can damage reputation. Customer.io

What I'd do at your size

2k–10k:
Klaviyo or Customer.io. Don't over-engineer the infrastructure.

10k–50k:
Start taking deliverability seriously: domain reputation, segmentation, engagement-based sending, suppression rules, and monitoring.

50k–100k:
I'd stay with Klaviyo if you're DTC/e-commerce, or Customer.io if you're SaaS/product-led. Consider dedicated infrastructure/IPs only when your sending volume and reputation justify it—not simply because your list hit 100k.

100k+ with serious enterprise volume:
Braze, Iterable, or a more customized Customer.io/email-provider architecture become increasingly attractive.

Bottom line:

• 🛒 E-commerce/DTC → Klaviyo
• 💻 SaaS/product-led → Customer.io
• 🏢 Enterprise/complex omnichannel → Braze
• 🚀 If you want the simplest path from 3k → 100k → Klaviyo is my default choice

If you tell me what kind of business you have, roughly how many emails/SMS you expect to send per month, and whether the 100k are mostly leads or customers, I can narrow this down to the best 2 and compare their estimated monthly cost at 10k, 50k, and 100k subscribers.

10k–50k:
Start taking deliverability seriously: domain reputation, segmentation, engagement-based sending, suppression rules, and monitoring.

50k–100k:
I'd stay with Klaviyo if you're DTC/e-commerce, or Customer.io if you're SaaS/product-led. Consider dedicated infrastructure/IPs only when your sending volume and reputation justify it—not simply because your list hit 100k.

100k+ with serious enterprise volume:
Braze, Iterable, or a more customized Customer.io/email-provider architecture become increasingly attractive.

Bottom line:

• 🛒 E-commerce/DTC → Klaviyo
• 💻 SaaS/product-led → Customer.io
• 🏢 Enterprise/complex omnichannel → Braze
• 🚀 If you want the simplest path from 3k → 100k → Klaviyo is my default choice

If you tell me what kind of business you have, roughly how many emails/SMS you expect to send per month, and whether the 100k are mostly leads or customers, I can narrow this down to the best 2 and compare their estimated monthly cost at 10k, 50k, and 100k subscribers.

The downside is that it's generally overkill for a business with only a few thousand subscribers, both operationally and financially.

The important part: platform ≠ deliverability

At 100k subscribers, your sending practices will matter more than which of these four you pick.

I'd prioritize:

If your goal is a few thousand → ~100,000 subscribers while protecting deliverability, I’d shortlist Klaviyo, Braze, and Customer.io. My default choice depends heavily on whether you’re ecommerce or SaaS.

My ranking

🥇 My pick: Klaviyo

For most businesses going from a few thousand to 100k, I'd choose Klaviyo unless you have unusually sophisticated product/customer-event requirements.

Its big advantage is that email and SMS are designed to work together rather than bolted together. More importantly, Klaviyo gives you the infrastructure and controls you'll want as volume grows: branded sending domains, authentication, reputation management, segmentation, automated flows, and SMS infrastructure. Klaviyo specifically recommends branded sending domains for accounts over 5,000 profiles, and recommends considering dedicated IPs when consistently sending over 1 million emails/month. Klaviyo Academy Klaviyo Help Center

For SMS, Klaviyo supports toll-free numbers and dedicated short codes; it notes that short codes are particularly appropriate when sending to 100k+ people per month because of their much higher throughput. Klaviyo

The sweet spot: ecommerce, subscriptions, consumer brands, newsletters + offers, lifecycle marketing.

🥈 Braze if you expect to become sophisticated

braze.com is what I'd choose if 100k subscribers is just the beginning and you expect to build a very sophisticated lifecycle/engagement operation.

Braze supports email, SMS/RCS, push, in-app, web, WhatsApp and other channels from the same platform. Its email offering includes customized IP-warming plans and deliverability specialists, while its SMS infrastructure is designed for high-volume programs. Braze Braze

Braze is also operating at enormous scale—the company says its platform averages 4.9M messaging/Canvas actions per minute and 14.3M API calls per minute. Braze

The downside is complexity and price. At only 5k–20k subscribers, Braze can be overkill.

🥉 Customer.io for SaaS

If you're a SaaS/product company where the important events are things like:

signed up → completed onboarding → invited teammate → used feature X → hit usage limit → upgraded

I'd seriously consider customer.io.

It's particularly good when messaging is driven by product behavior rather than primarily by ecommerce events.

The more important issue: deliverability

At 100k subscribers, the platform isn't what determines whether you land in the inbox. Your sending practices matter enormously.

I'd build the stack around:

• Branded sending domain rather than the ESP's generic domain.
• SPF + DKIM + DMARC correctly configured.
• Gradual volume ramping rather than suddenly mailing 100k people.
• Aggressive suppression of chronically unengaged subscribers.
• Separate transactional and marketing email streams.
• Consistent sending patterns.
• Careful management of Gmail/Yahoo/Microsoft reputation.
• Double opt-in or otherwise very high-quality acquisition.
• Avoiding purchased/rented lists.
• Separate SMS consent and compliance processes.

Klaviyo's own deliverability guidance emphasizes that your sending domain, IP, and click-tracking domain each have reputations, and recommends branded sending domains and proper authentication. Klaviyo Academy Klaviyo Help Center

One interesting independent 2026 benchmark also puts Klaviyo and Braze near the top for deliverability, although I wouldn't treat a single benchmark as definitive. Loops

What I'd do in your position

Ecommerce/DTC: → Klaviyo

SaaS/app: → Customer.io

Already sophisticated / millions of events / multiple channels: → Braze

Need CRM + marketing + sales all together: → HubSpot

If you tell me what kind of business you have, your current subscriber count, roughly how many emails/SMS you'll send per month, and whether most subscribers are US-based, I can narrow this down to one platform and explain what the architecture should look like from 5k → 100k.

Braze is also operating at enormous scale—the company says its platform averages 4.9M messaging/Canvas actions per minute and 14.3M API calls per minute. Braze

The downside is complexity and price. At only 5k–20k subscribers, Braze can be overkill.

🥉 Customer.io for SaaS

If you're a SaaS/product company where the important events are things like:

signed up → completed onboarding → invited teammate → used feature X → hit usage limit → upgraded

I'd seriously consider customer.io.

It's particularly good when messaging is driven by product behavior rather than primarily by ecommerce events.

The more important issue: deliverability

At 100k subscribers, the platform isn't what determines whether you land in the inbox. Your sending practices matter enormously.

I'd build the stack around:

• Branded sending domain rather than the ESP's generic domain.
• SPF + DKIM + DMARC correctly configured.
• Gradual volume ramping rather than suddenly mailing 100k people.
• Aggressive suppression of chronically unengaged subscribers.
• Separate transactional and marketing email streams.
• Consistent sending patterns.
• Careful management of Gmail/Yahoo/Microsoft reputation.
• Double opt-in or otherwise very high-quality acquisition.
• Avoiding purchased/rented lists.
• Separate SMS consent and compliance processes.

Klaviyo's own deliverability guidance emphasizes that your sending domain, IP, and click-tracking domain each have reputations, and recommends branded sending domains and proper authentication.

If your goal is a few thousand → 100k+ subscribers while keeping deliverability strong, I’d shortlist Klaviyo, Customer.io, and Braze. The best choice depends heavily on whether you’re e-commerce, SaaS, or a media/content business.

My ranking

1. klaviyo.com — my default recommendation

If you're building a consumer brand, newsletter + commerce business, or DTC company, I'd probably start here.

Klaviyo has email and SMS under one platform, strong segmentation/flows, and dedicated deliverability tooling. Its current deliverability hub monitors both email and SMS health. Klaviyo Help Center Klaviyo Help Center

It's also designed to handle significant SMS volume; for example, Klaviyo notes that dedicated short codes can provide extremely high throughput when you're sending to 100k+ people/month. Klaviyo

Why I'd choose it: you can start relatively simply at 2–5k subscribers without building a complicated messaging stack, then grow into sophisticated segmentation and automated flows.

2. customer.io — best for SaaS/product-led businesses

I'd choose Customer.io over Klaviyo if your subscribers/users have lots of behavioral events:

signed up → used feature → abandoned onboarding → upgraded → hasn't logged in → etc.

Customer.io is particularly flexible because you can use its email infrastructure or bring your own provider, and its SMS layer supports Twilio, Sinch, Infobip, and Vonage. In the US/Canada, it can also manage SMS sending directly. Customer.io Customer.io

It also offers shared and dedicated IP options, including a stricter transactional IP pool. Customer.io

Big advantage: you're less locked into one underlying delivery provider.

3. braze.com — best if you know you're going big

Braze is what I'd investigate if 100k is merely the beginning and you expect hundreds of thousands or millions of users.

Braze explicitly supports high-volume email with customized IP-warming plans and deliverability/ISP expertise. Its infrastructure currently handles millions of messaging/Canvas actions per minute. Braze Braze

The tradeoff is that it's considerably more enterprise-oriented. For a company with 3,000 subscribers, it can be overkill.

The important part: the platform isn't what determines deliverability

This is the biggest misconception I'd avoid.

Going from 5k → 100k doesn't inherently destroy deliverability. Bad list acquisition and sending practices do.

Regardless of platform, I'd set up:

• Dedicated sending subdomain, e.g. mail.yourdomain.com
• SPF, DKIM and DMARC authentication
• Gradual volume ramping rather than immediately blasting 100k people
• Double opt-in or very strong consent capture
• Engagement-based segmentation
• Automatic suppression/sunset of chronically inactive subscribers
• Separate promotional and transactional sending identities
• Continuous monitoring of bounces, spam complaints and engagement
• SMS consent that is explicitly collected separately from email consent

For example, Customer.io currently recommends authentication, double opt-in, engagement filtering and sunset policies as core deliverability practices. Customer.io Customer.io

And this matters particularly during a platform migration: changing ESPs changes parts of your sender identity, so you shouldn't simply import 100k addresses and immediately send to everyone. Customer.io

What I'd pick

If you're e-commerce/DTC:
Klaviyo + Klaviyo SMS

If you're SaaS/app/subscription:
Customer.io + Twilio (or another SMS provider)

If you're already substantial and expect 500k–millions:
Braze

If you're starting at 2–5k and don't know where you'll end up:
I'd lean Klaviyo for simplicity or Customer.io if your product generates lots of behavioral data.

One useful independent 2026 benchmark also puts Klaviyo and Braze near the top for deliverability, with Customer.io close behind, although I wouldn't treat any single third-party benchmark as definitive. Loops

If you tell me what kind of business this is (e-commerce, SaaS, newsletter/media, course, etc.), your expected monthly email volume, and whether SMS is US-only or international, I can narrow this down to the best 1–2 platforms and estimate what the 100k-subscriber stack would cost.

It also offers shared and dedicated IP options, including a stricter transactional IP pool. Customer.io

Big advantage: you're less locked into one underlying delivery provider.

3. braze.com — best if you know you're going big

Braze is what I'd investigate if 100k is merely the beginning and you expect hundreds of thousands or millions of users.

Braze explicitly supports high-volume email with customized IP-warming plans and deliverability/ISP expertise. Its infrastructure currently handles millions of messaging/Canvas actions per minute. braze.com braze.com

The tradeoff is that it's considerably more enterprise-oriented. For a company with 3,000 subscribers, it can be overkill.

The important part: the platform isn't what determines deliverability

This is the biggest misconception I'd avoid.

Going from 5k → 100k doesn't inherently destroy deliverability. Bad list acquisition and sending practices do.

If your goal is a few thousand → 100k+ subscribers while protecting email deliverability and having SMS in the same system, I’d shortlist these:

🥇 My default recommendation: Klaviyo

For 2k → 100k subscribers, I'd generally start with Klaviyo unless you have unusually complex product/event data.

The important thing is that Klaviyo's deliverability isn't simply a consequence of having a big infrastructure provider. You still need to build your own sender/domain reputation. Klaviyo recommends a branded sending domain, and its current guidance says accounts over 5,000 profiles should use one with DMARC. Klaviyo Academy Klaviyo Help Center

At 100k subscribers, you also don't necessarily need a dedicated IP. Klaviyo currently recommends dedicated IPs primarily when you're consistently sending >1 million emails/month. Klaviyo Help Center

For SMS, Klaviyo supports toll-free numbers and dedicated short codes; its documentation specifically notes short codes as a good option when you're sending SMS to 100k+ people/month. Klaviyo

klaviyo.com

🥈 When I'd choose Braze instead

If you're building something that will eventually have 100k → millions of users, multiple channels, sophisticated behavioral journeys, mobile push, in-app messaging, RCS, etc., Braze is probably the stronger long-term platform.

Braze explicitly provides email and SMS/RCS in the same platform and has dedicated deliverability services including IP warming, monitoring, and deliverability specialists. Braze Braze

The downside is that it's much more of an enterprise customer-engagement platform than a simple marketing tool. I'd hesitate to introduce that complexity when you're only at a few thousand subscribers.

braze.com

🥉 Customer.io is particularly interesting for SaaS

If you're running a SaaS/product business where messaging is triggered by things like:

signed up → completed onboarding → used feature X → hasn't logged in for 14 days → upgraded

I'd seriously consider Customer.io.

It has strong event-based journey capabilities and explicit deliverability controls. Its current documentation recommends authentication, engagement filtering and sunset policies, and it maintains separate transactional IP infrastructure. Customer.io Customer.io

One thing I'd be aware of: Customer.io currently applies sending limits/warm-up periods to new accounts, so you want to plan the migration and volume ramp rather than dumping 100k contacts into a brand-new sender and immediately blasting them. Customer.io Customer.io

customer.io

The bigger deliverability point

The platform matters less than your sending practices.

For 100k subscribers, I'd build the system around:

• A dedicated sending subdomain (e.g. email.yourbrand.com)
• SPF + DKIM + DMARC correctly configured
• Gradual domain/IP warming
• Separate marketing vs. transactional sending
• Aggressive suppression of chronically unengaged subscribers
• Double opt-in or strong signup validation
• Monitoring Gmail/Yahoo/Microsoft placement and complaints
• Never buying/importing questionable lists
• Gradually increasing volume rather than going from 5k to 100k overnight

Customer.io's current guidance, for example, says complaint rates above 0.10% can damage sender reputation, illustrating how little tolerance mailbox providers have for poor engagement. Customer.io

What I'd pick

E-commerce/DTC: Klaviyo
SaaS/product-led: Customer.io
Large enterprise / millions of users / many channels: Braze
Enterprise marketing organization: Iterable or Braze

If you tell me what kind of business you have, roughly how many emails/SMS you'll send per month, and whether the 100k are mostly email subscribers or actual customers, I can narrow this down to the best 1–2 options and compare pricing + deliverability + migration risk at 100k.

klaviyo.com

🥈 When I'd choose Braze instead

If you're building something that will eventually have 100k → millions of users, multiple channels, sophisticated behavioral journeys, mobile push, in-app messaging, RCS, etc., Braze is probably the stronger long-term platform.

Braze explicitly provides email and SMS/RCS in the same platform and has dedicated deliverability services including IP warming, monitoring, and deliverability specialists. Braze Braze

The downside is that it's much more of an enterprise customer-engagement platform than a simple marketing tool. I'd hesitate to introduce that complexity when you're only at a few thousand subscribers.

braze.com

🥉 Customer.io is particularly interesting for SaaS

If you're running a SaaS/product business where messaging is triggered by things like:

signed up → completed onboarding → used feature X → hasn't logged in for 14 days → upgraded

I'd seriously consider Customer.io.

It has strong event-based journey capabilities and explicit deliverability controls. Its current documentation recommends authentication, engagement filtering and sunset policies, and it maintains separate transactional IP infrastructure. Customer.io Customer.io

One thing I'd be aware of: Customer.io currently applies sending limits/warm-up periods to new accounts, so you want to plan the migration and volume ramp rather than dumping 100k contacts into a brand-new sender and immediately blasting them. Customer.io Customer.io

customer.io

The bigger deliverability point

The platform matters less than your sending practices.

For 100k subscribers, I'd build the system around:

If your goal is a few thousand → ~100,000 subscribers while protecting deliverability, I’d shortlist Klaviyo, Braze, and Attentive. My default pick for most growing businesses is Klaviyo.

🥇 My pick: Klaviyo

klaviyo.com is probably the best growth-stage choice if you're starting with a few thousand contacts and expect to reach 100k.

The important thing is that 100k subscribers itself isn't a difficult scale threshold. Deliverability is much more dependent on your sending reputation, authentication, engagement, list hygiene, and sending patterns.

Klaviyo specifically recommends a branded sending domain and supports DKIM/SPF/DMARC configuration. It also recommends dedicated IP infrastructure only once sending volume is sufficiently high; its current guidance says dedicated IPs generally make sense around 1M+ emails/month, rather than simply because you have 100k subscribers. Klaviyo Academy Klaviyo Help Center Klaviyo Help Center

For SMS, Klaviyo can keep email and SMS journeys/segmentation together, which is a major advantage as your list grows. Its infrastructure supports higher-throughput SMS options as your volume increases. Klaviyo

🥈 Braze if you're building something much more sophisticated

braze.com is what I'd choose if you expect to become a large, sophisticated lifecycle-marketing operation, rather than simply a 100k-subscriber newsletter/ecommerce brand.

Braze has particularly strong cross-channel orchestration across email, SMS, push, in-app, web, etc. Its email infrastructure includes customized IP warming, monitoring, and deliverability specialists. Braze Braze

The downside is that it's generally more enterprise-oriented and complex than you need at 3k–10k subscribers.

🥉 Attentive if SMS is central to the business

attentive.com is worth serious consideration if SMS is going to be a huge portion of your revenue.

It combines email + SMS with strong identity resolution and hands-on deliverability support. Attentive currently positions itself particularly toward larger consumer brands and reports 99.8%+ deliverability on its platform. Attentive Attentive

What I'd actually do

For a company going:

3k → 10k → 25k → 50k → 100k subscribers

I'd start with Klaviyo, rather than migrating platforms halfway through that growth.

The more important investment is setting up the infrastructure correctly from day one:

• Use your own branded sending domain
• Configure SPF, DKIM and DMARC
• Warm the domain gradually
• Don't blast your entire database just because you have 100k people
• Segment heavily based on engagement
• Automatically suppress chronically unengaged subscribers
• Keep acquisition sources clean—never buy lists
• Separate marketing and transactional email infrastructure
• For SMS, maintain rigorous opt-in/opt-out compliance
• Monitor Gmail/Yahoo/Microsoft reputation separately
• Increase volume gradually when your engagement is strong

Klaviyo's current guidance explicitly emphasizes branded-domain authentication, warming, engagement-based sending, and list hygiene as core deliverability practices. Klaviyo Help Center Klaviyo Help Center

One important caveat: if you're not ecommerce/DTC, my recommendation could change substantially. For example, I'd lean Customer.io for a SaaS/product with highly behavioral messaging, and Braze if you're building a large multi-channel lifecycle operation.

If you tell me what kind of business you have (ecommerce, SaaS, newsletter/media, info product, etc.) and roughly how many emails/SMS you expect to send per month, I can give you a much more specific winner—including Klaviyo vs Braze vs Customer.io vs Attentive on cost at 100k.

For SMS, Klaviyo can keep email and SMS journeys/segmentation together, which is a major advantage as your list grows. Its infrastructure supports higher-throughput SMS options as your volume increases. klaviyo.com

🥈 Braze if you're building something much more sophisticated

braze.com is what I'd choose if you expect to become a large, sophisticated lifecycle-marketing operation, rather than simply a 100k-subscriber newsletter/ecommerce brand.

Braze has particularly strong cross-channel orchestration across email, SMS, push, in-app, web, etc. Its email infrastructure includes customized IP warming, monitoring, and deliverability specialists. braze.com braze.com

The downside is that it's generally more enterprise-oriented and complex than you need at 3k–10k subscribers.

🥉 Attentive if SMS is central to the business

If your goal is a few thousand → 100k subscribers without sacrificing deliverability, I’d shortlist Klaviyo, Customer.io, and Braze. My default recommendation for most companies is Klaviyo.

My ranking

🥇 Best overall: Klaviyo

I'd choose Klaviyo if you're starting around a few thousand subscribers and expect to reach 100k.

The biggest reason isn't simply "Klaviyo delivers email well." It's that you can start relatively simply and progressively build a serious sending infrastructure as volume increases.

Klaviyo recommends a branded sending domain once you're above 5,000 active profiles or sending to 5,000 people in a day. That lets your domain build its own reputation rather than relying entirely on a shared sending domain. Klaviyo Academy Klaviyo Help Center

Importantly, 100k subscribers does not automatically mean you need a dedicated IP. Klaviyo currently recommends dedicated IPs primarily for senders consistently exceeding about 1 million emails/month; shared infrastructure can actually be preferable for smaller/inconsistent senders. Klaviyo Help Center

Klaviyo also has mature SMS infrastructure, including toll-free numbers and dedicated short codes for very high-volume SMS programs. Klaviyo

klaviyo.com

🥈 Customer.io — best if you're SaaS/product-led

I'd pick Customer.io over Klaviyo if your messaging is driven heavily by user behavior inside an app:

• "User signed up but hasn't activated"
• "User completed X but not Y"
• complex behavioral journeys
• event-based lifecycle messaging
• sophisticated segmentation

Customer.io lets you use its managed email infrastructure or connect your own delivery provider, which gives you more flexibility as you mature. Customer.io

It also has separate transactional IP infrastructure with stricter bounce/spam thresholds, which is attractive when transactional and marketing traffic become important. Customer.io

customer.io

🥉 Braze — best if you're building an enterprise messaging stack

Braze is extremely capable for email + SMS + push + in-app + sophisticated customer journeys.

Its deliverability infrastructure is particularly strong at high scale: Braze offers customized IP-warming plans, monitoring, and deliverability specialists. Braze

The downside is that Braze is usually overkill at 2–10k subscribers. I'd consider it when you're building a much larger lifecycle/engagement operation rather than simply looking for a platform that can grow to 100k.

braze.com

The important part: platform isn't what determines deliverability

At 100k subscribers, your sending practices matter at least as much as the vendor.

I'd set it up like this:

Email

• Branded sending domain
• SPF + DKIM + DMARC
• Separate marketing and transactional sending domains if appropriate
• Gradual volume ramp when migrating
• Aggressive suppression of chronically unengaged users
• Monitor Gmail/Yahoo/Microsoft separately
• Avoid suddenly mailing all 100k because your database reached 100k

SMS

• Proper consent/opt-in
• Separate transactional vs promotional logic
• Automatic suppression after opt-out
• Monitor carrier filtering
• Use toll-free initially; consider a short code as volume becomes substantial

Klaviyo itself emphasizes domain authentication, warming, list hygiene, and engagement as core components of deliverability—not simply the choice of IP. Klaviyo Help Center Klaviyo Help Center

What I'd choose

E-commerce/DTC: Klaviyo

SaaS/app: Customer.io

Enterprise / millions of users / complex omnichannel: Braze

If you're specifically asking "I have ~3–10k now and expect to reach 100k; what's the best combination of price + deliverability + email/SMS + ease of scaling?", I'd lean Klaviyo unless there's a strong SaaS/product-event reason to choose Customer.io.

One caveat: 100k subscribers is not the same as 100k monthly recipients. If you tell me roughly how many emails and SMS messages you expect to send per month, I can compare the likely economics of Klaviyo vs Customer.io vs Braze at 10k, 25k, 50k, and 100k.

If your goal is a few thousand → ~100,000 subscribers while maintaining strong email + SMS deliverability, my default pick would be Klaviyo—especially if you're doing ecommerce, DTC, subscriptions, or consumer marketing.

My ranking

1. 🥇 Klaviyo — best default choice

klaviyo.com

I'd choose Klaviyo if you want one platform for email + SMS and don't want to rebuild your messaging stack when you hit 50k–100k people.

The important thing is that Klaviyo isn't just capable of sending large volumes; it has substantial deliverability tooling. Its current deliverability system monitors email and SMS health, and Klaviyo recommends dedicated sending domains so your brand builds reputation on its own domain. Klaviyo Help Center Klaviyo Help Center Klaviyo Help Center

For SMS, Klaviyo has dedicated deliverability reporting and infrastructure designed for high-volume sending. It also supports toll-free numbers and dedicated short codes; Klaviyo specifically notes short codes as an option for audiences sending to 100k+ people per month. Klaviyo Help Center Klaviyo

Why I'd pick it: you can start relatively simply and progressively add segmentation, flows, behavioral triggers, SMS, dedicated sending infrastructure, etc., without migrating platforms at 25k or 100k subscribers.

2. 🥈 Customer.io — better for SaaS/product lifecycle

customer.io

I'd pick Customer.io over Klaviyo if your subscribers are really users of a product/app, rather than primarily shoppers.

It's particularly strong when you want things like:

User signs up → doesn't complete onboarding → receives email → performs action → receives SMS → upgrades → enters a different lifecycle.

Customer.io gives you sophisticated event-based segmentation and automation, and you can eventually use your own SMTP provider if you need greater control over email infrastructure. Customer.io Customer.io

Its deliverability infrastructure is also serious: it monitors shared IP pools and offers dedicated/transactional IP options. Customer.io

One thing I'd watch: new accounts are intentionally ramped up with sending limits as Customer.io establishes a healthy sending history. That's good for deliverability, but you'll need to plan your initial migration/warm-up rather than dumping 100k contacts into a new account and immediately blasting them. Customer.io

3. 🥉 Braze — excellent, but start here only if you're already sophisticated

braze.com

Braze is the choice I'd consider if you expect your messaging operation to become very large and very sophisticated.

Its email infrastructure includes customized IP warming, monitoring, and deliverability specialists. Braze Braze

But for going from 3k → 100k subscribers, I wouldn't necessarily pay the complexity/enterprise overhead unless you already have a sophisticated product, data infrastructure, and lifecycle-marketing team.

The bigger issue: the platform isn't what determines deliverability

This is the part I'd emphasize.

Klaviyo with a bad sending strategy will perform worse than Customer.io with a good one.

At 100k subscribers, I'd build your system around:

• Dedicated sending domain/subdomain
• SPF + DKIM + DMARC properly configured
• Gradual volume increases
• Separate marketing and transactional email streams
• Strong suppression/bounce handling
• Automatic sunset of chronically unengaged subscribers
• Double opt-in where appropriate
• Aggressive spam-complaint monitoring
• Behavioral segmentation instead of blasting the entire database
• Separate SMS consent from email consent
• Proper toll-free/short-code registration and carrier compliance
• Careful migration/warming if you change ESPs

Customer.io, for example, warns that complaint rates above 0.10% can damage IP/domain reputation, and recommends engagement filtering and sunset policies. Customer.io

And Klaviyo similarly emphasizes infrastructure, warming, list hygiene, monitoring, and testing as core parts of deliverability—not merely choosing its platform. Klaviyo Help Center

What I'd actually do

If you told me:

"We're at 3,000 subscribers today and expect 100,000 within 12–24 months."

I'd choose:

Ecommerce/DTC → Klaviyo

SaaS/app/product → Customer.io

Large enterprise / complex omnichannel → Braze

And I would not optimize for the absolute cheapest cost per thousand messages at this stage. A platform that's $500/month cheaper but causes you to lose 5–10% of your valuable email audience to poor inbox placement is an extremely expensive bargain.

If you tell me what kind of business this is (ecommerce, SaaS, newsletter/media, B2B, etc.) and roughly how many emails/SMS you'll send per month, I can narrow this down to the best 2–3 options and compare pricing at 10k, 50k, and 100k subscribers, including the likely infrastructure you'd want for deliverability.

User signs up → doesn't complete onboarding → receives email → performs action → receives SMS → upgrades → enters a different lifecycle.

Customer.io gives you sophisticated event-based segmentation and automation, and you can eventually use your own SMTP provider if you need greater control over email infrastructure. Customer.io Customer.io

Its deliverability infrastructure is also serious: it monitors shared IP pools and offers dedicated/transactional IP options. Customer.io

One thing I'd watch: new accounts are intentionally ramped up with sending limits as Customer.io establishes a healthy sending history. That's good for deliverability, but you'll need to plan your initial migration/warm-up rather than dumping 100k contacts into a new account and immediately blasting them. Customer.io

3. 🥉 Braze — excellent, but start here only if you're already sophisticated

braze.com

Braze is the choice I'd consider if you expect your messaging operation to become very large and very sophisticated.

Its email infrastructure includes customized IP warming, monitoring, and deliverability specialists.

But for going from 3k → 100k subscribers, I wouldn't necessarily pay the complexity/enterprise overhead unless you already have a sophisticated product, data infrastructure, and lifecycle-marketing team.

The bigger issue: the platform isn't what determines deliverability

This is the part I'd emphasize.

For scaling from a few thousand → 100k+ subscribers, the “best” platform depends heavily on whether you are ecommerce, SaaS/app, media/community, or B2B, but for most companies the shortlist is fairly clear.

The biggest factor for deliverability is not just the vendor — it is the combination of:

• sender reputation management
• authentication (SPF, DKIM, DMARC)
• list hygiene
• segmentation
• gradual volume ramping
• engagement-based sending

A great platform can still get poor inbox placement with bad practices.

Best overall choices

Product Growth Klaviyo## My recommendations by business type

If you are ecommerce (Shopify, subscriptions, DTC)

Pick: Klaviyo

Why:

• Best-in-class behavioral segmentation
• Strong email + SMS combination
• Excellent abandoned cart, winback, replenishment flows
• Easy migration path from 5k → 100k contacts

Klaviyo is widely used by ecommerce brands because it combines customer data, email, SMS, and automation in one system. Klaviyo Klaviyo

Typical stack:

• Klaviyo → marketing email + SMS
• postscript.io or Klaviyo SMS → heavy SMS programs
• aws.amazon.com or sendgrid.com → transactional email (optional)

If you are SaaS / app / membership product

Pick: Customer.io

Why:

• Event-based messaging (“user did X → send Y”)
• Better for product lifecycle than newsletters
• Handles complex journeys well
• Developer-friendly

Example flows:

• Day 1 onboarding
• Feature adoption
• Trial conversion
• Churn prevention
• Usage-based messaging

Customer.io is often compared with Braze and Klaviyo as a lifecycle engagement platform, with a more developer-oriented approach. Product Growth Taqtics

If you expect millions of users eventually

Pick: Braze

Good fit for:

• mobile apps
• marketplaces
• fintech
• gaming
• large consumer platforms

Strengths:

• Email
• SMS
• push notifications
• in-app messaging
• complex customer journeys

The downside is cost and implementation complexity. Product Growth Maestra

Platforms I would be cautious about for your goal

Mailchimp

Fine for early newsletters, but many companies outgrow it once they need:

• advanced segmentation
• complex automation
• behavioral triggers
• sophisticated deliverability management

Cheap “all-in-one” email tools

They can work at 5k subscribers but often become painful at 100k because:

• weaker analytics
• fewer deliverability controls
• limited automation logic

What I would choose for a 5k → 100k growth path

Consumer brand:

Klaviyo

SaaS:

Customer.io

Mobile app with aggressive growth:

Braze

Newsletter/media business:

A combination of:

• beehiiv.com or convertkit.com for publishing
• aws.amazon.com / sendgrid.com for high-volume sending if needed

Deliverability checklist before you hit 100k

Regardless of platform:

1. Set up:

• SPF
• DKIM
• DMARC (start with monitoring, then enforce)
• Separate streams:

• marketing email domain/subdomain
• transactional email domain/subdomain
• Never blast inactive users:

• suppress people who have not opened/clicked for months
• use re-engagement campaigns
• Warm gradually:

• don't go from 5k sends/week to 500k overnight
• Track:

• inbox placement
• spam complaints
• bounce rate
• engagement by segment

For most companies starting at a few thousand subscribers and aiming for 100k, I would avoid overbuilding early: Klaviyo for ecommerce or Customer.io for SaaS gives the best balance of deliverability, automation, and scalability. Product Growth Klaviyo

If you tell me your business type (ecommerce, SaaS, newsletter, marketplace, etc.) and whether SMS is a core channel or just a backup, I can narrow it to a specific stack.

A combination of:

• beehiiv.com or convertkit.com for publishing
• aws.amazon.com / sendgrid.com for high-volume sending if needed

Deliverability checklist before you hit 100k

Regardless of platform:

1. Set up:

• SPF
• DKIM
• DMARC (start with monitoring, then enforce)
• Separate streams:

• marketing email domain/subdomain
• transactional email domain/subdomain
• Never blast inactive users:

• suppress people who have not opened/clicked for months
• use re-engagement campaigns
• Warm gradually:

• don't go from 5k sends/week to 500k overnight
• Track:

• inbox placement
• spam complaints
• bounce rate
• engagement by segment

For most companies starting at a few thousand subscribers and aiming for 100k, I would avoid overbuilding early: Klaviyo for ecommerce or Customer.io for SaaS gives the best balance of deliverability, automation, and scalability.

If your goal is a few thousand → 100k+ subscribers while keeping email deliverability and SMS performance strong, my default pick is Klaviyo—especially for e-commerce/DTC. For SaaS or highly event-driven products, I’d look harder at Customer.io or Braze.

My ranking

🥇 Best overall: Klaviyo

klaviyo.com

I'd choose Klaviyo if you're building a consumer brand, newsletter + commerce business, subscription business, or DTC operation.

The important thing is that Klaviyo isn't just "good at sending lots of emails." Its infrastructure lets you establish a branded sending domain, which becomes important once you're above roughly 5,000 active profiles. Klaviyo specifically recommends branded sending domains and DMARC for larger senders. Klaviyo Academy Klaviyo Help Center

For 100k subscribers, I would configure:

• Branded sending domain
• SPF + DKIM + DMARC
• Branded click-tracking domain
• Gradual volume increases rather than immediately blasting the whole database
• Aggressive suppression of unengaged users
• Separate transactional and marketing infrastructure where appropriate
• Shared IP initially; consider dedicated IP only once your volume warrants it

Klaviyo currently recommends dedicated IPs primarily for accounts consistently sending 1M+ emails/month, so I would not automatically buy a dedicated IP just because you reach 100k subscribers. Klaviyo Help Center Klaviyo Help Center

For SMS, Klaviyo is particularly attractive because email and SMS live in the same customer/segmentation system. Its SMS infrastructure supports toll-free numbers and dedicated short codes; Klaviyo says short codes are appropriate when you're sending to 100k+ people per month. Klaviyo

🥈 Customer.io — my pick for SaaS

customer.io

If you're a SaaS/product company rather than a store, Customer.io may actually be the better choice.

Its strength is event-driven messaging:

user signs up → onboarding → uses feature → doesn't use feature → upgrade prompt → trial ending → retention campaign

It supports email and SMS within the same workflow, with Twilio as its default SMS provider and support for other providers. Customer.io

Customer.io also takes deliverability seriously: its current documentation recommends authentication, gradual sending, suppression-list migration, and engagement-based warming when moving platforms. Customer.io Customer.io

One thing I'd pay attention to: don't migrate 50k–100k people and immediately send to everyone. Customer.io explicitly recommends starting with your most engaged recipients and ramping volume. Customer.io

🥉 Braze — if you're going much bigger

braze.com

Braze is excellent if you're building toward hundreds of thousands or millions of users and need sophisticated omnichannel orchestration.

But for 5k → 100k subscribers, it can be overkill. You're paying for enterprise-grade capabilities that you may not need yet.

I'd consider Braze if you're expecting:

• 500k–1M+ users
• Multiple countries
• Email + SMS + push + in-app + WhatsApp
• Complex real-time behavioral journeys
• Dedicated marketing/CRM engineering resources

What I'd actually do

If you told me:

"We're at 3,000 subscribers today and expect 100,000 within 18–24 months."

I'd choose:

E-commerce/DTC → Klaviyo

SaaS/app → Customer.io

Enterprise/mobile app → Braze

And I wouldn't choose the platform primarily based on advertised "deliverability rates." Your domain reputation, authentication, complaint rate, engagement, list acquisition practices, and sending patterns matter enormously. Customer.io, for example, notes that complaint rates above 0.10% can damage sender reputation. Customer.io

The biggest mistake is getting to 100k and treating all 100k subscribers as equally valuable. A clean, engaged 50k list will generally be much healthier to send to than a 100k list full of dormant addresses.

If you tell me what kind of business this is (e-commerce, SaaS, newsletter, creator, etc.) and roughly how many emails/SMS you'll send per month, I can give you a much more specific recommendation—including Klaviyo vs Customer.io vs Braze, estimated cost at 10k/50k/100k subscribers, and the infrastructure I'd use for maximum deliverability.

If your goal is a few thousand → ~100,000 subscribers while keeping email deliverability high and adding SMS, I’d shortlist Klaviyo, Customer.io, and Braze.

My ranking

🥇 My default choice: Klaviyo

If you're primarily doing marketing campaigns, automated flows, ecommerce, newsletters, promotions, and SMS, I'd choose Klaviyo.

The important thing is that Klaviyo's current guidance doesn't suggest jumping to a dedicated IP merely because you hit 100k subscribers. It recommends a branded sending domain + proper authentication, while dedicated IPs become more relevant at consistently high sending volumes; Klaviyo currently recommends dedicated IPs around 1M+ emails/month. Klaviyo Help Center Klaviyo Help Center

That makes it a particularly good fit for your growth path: you can start on shared infrastructure, establish your domain reputation, and move to dedicated infrastructure when your actual sending volume warrants it.

Klaviyo also supports both email and SMS in one system, including higher-throughput SMS options when your volume becomes substantial. Klaviyo

klaviyo.com

🥈 Customer.io — my pick for SaaS

I'd choose Customer.io over Klaviyo if your subscribers are really product users rather than primarily shoppers.

It's particularly strong when you need things like:

• event-triggered messaging
• complex lifecycle journeys
• behavioral segmentation
• product usage data
• transactional + marketing messaging
• developer/API control

Customer.io currently offers managed shared IP pools, transactional IP pools, and dedicated IPs. Its dedicated-IP threshold is 50,000 emails/week, with Customer.io handling the IP warming process. Customer.io Customer.io

It also explicitly monitors bounce, complaint and blocklist activity and provides deliverability expertise. Customer.io

customer.io

🥉 Braze — if you're building a serious enterprise lifecycle stack

Braze is arguably the most powerful of the three, but I wouldn't pay for that complexity unless you need it.

It's excellent for sophisticated omnichannel programs involving email, SMS, push, in-app messaging, experimentation and complex customer journeys. Braze specifically offers customized IP-warming plans, monitoring, and deliverability specialists for high-scale sending. Braze Braze

braze.com

The bigger deliverability point

The platform is only part of the equation. At 100k subscribers, your sending practices matter more than whether you picked Klaviyo vs. Customer.io.

I'd set things up like this:

Your brand domain
→ authenticated sending domain
→ SPF + DKIM + DMARC
→ separate marketing/transactional streams
→ engagement-based segmentation
→ automatic suppression of inactive users
→ gradual volume increases
→ monitor Gmail/Yahoo/Outlook reputation

For example, Customer.io notes that complaint rates above 0.10% can damage sender reputation, and recommends double opt-in, engagement filtering and sunset policies. Customer.io

Also, 100k subscribers doesn't necessarily mean you need a dedicated IP. If you're emailing 100k people once a month, that's very different from emailing 100k people several times a week. Consistent sending volume and reputation are what matter.

What I'd choose

• E-commerce / DTC: Klaviyo
• SaaS / app / membership: Customer.io
• Large enterprise / complex omnichannel: Braze
• B2B where CRM is central: HubSpot
• Small-to-mid-market ecommerce where simplicity matters: Omnisend

If you tell me what kind of business you have (ecommerce, SaaS, newsletter/media, etc.) and roughly how many emails/SMS messages you expect to send per month, I can give you a much more specific recommendation—including estimated costs at 10k, 50k, and 100k subscribers.

🥉 Braze — if you're building a serious enterprise lifecycle stack

Braze is arguably the most powerful of the three, but I wouldn't pay for that complexity unless you need it.

It's excellent for sophisticated omnichannel programs involving email, SMS, push, in-app messaging, experimentation and complex customer journeys. Braze specifically offers customized IP-warming plans, monitoring, and deliverability specialists for high-scale sending. braze.com braze.com

braze.com

The bigger deliverability point

The platform is only part of the equation. At 100k subscribers, your sending practices matter more than whether you picked Klaviyo vs. Customer.io.

I'd set things up like this:

Your brand domain
→ authenticated sending domain
→ SPF + DKIM + DMARC
→ separate marketing/transactional streams
→ engagement-based segmentation
→ automatic suppression of inactive users
→ gradual volume increases
→ monitor Gmail/Yahoo/Outlook reputation

For example, Customer.io notes that complaint rates above 0.10% can damage sender reputation, and recommends double opt-in, engagement filtering and sunset policies.

If your goal is a few thousand → 100k subscribers while keeping deliverability strong, my default recommendation is Klaviyo—especially if you're ecommerce. If you're SaaS/product-led, I'd lean Customer.io instead.

My shortlist

Recent 2026 benchmark testing puts Klaviyo near the top for deliverability, while Customer.io and Braze also perform strongly. Loops Email Deliver Metrics

🥇 I'd choose Klaviyo for most businesses

Klaviyo is particularly compelling because email, SMS, segmentation, behavioral data, and automation are designed to work together. Independent 2026 testing found ~96.8% inbox placement for Klaviyo in a 100k-email test, and other recent benchmark data also ranks its sending capabilities highly. GetOmnichannel Loops Email

It's also explicitly designed around ecommerce use cases such as:

• Welcome sequences
• Abandoned cart
• Post-purchase
• Browse abandonment
• Win-back
• Product/customer segmentation
• Email + SMS journeys

Klaviyo's own 2026 guidance likewise highlights its unified email/SMS/push ecosystem and strong deliverability for ecommerce. Klaviyo

The important part: 100k subscribers itself isn't particularly scary for a mature ESP. What determines deliverability is much more about engagement, list hygiene, authentication, sending patterns, complaint rates, and segmentation.

If you're SaaS instead

I'd seriously consider Customer.io.

Customer.io is better when your important signals are things like:

signed up → completed onboarding → invited teammate → used feature → became inactive → upgraded

rather than:

viewed product → added to cart → purchased → reordered.

Customer.io's event-first architecture makes it particularly attractive for product-led businesses. Product Growth Intelligence CodeDrips

If you're already approaching enterprise scale

Braze becomes more interesting.

Braze is built for sophisticated cross-channel orchestration—email, SMS, push, in-app, etc.—and is a better long-term fit if you expect hundreds of thousands to millions of users, multiple messaging channels, and a dedicated marketing/engineering team. The tradeoff is substantially more complexity and cost. Klaviyo

What I'd actually do

If I were starting with 5k subscribers and planning for 100k, I'd avoid optimizing around the cheapest provider.

I'd optimize for:

1. Klaviyo — ecommerce
2. Customer.io — SaaS/product
3. Braze — if you know you'll become enterprise-scale

And I'd spend as much attention on the deliverability architecture as the ESP:

• Set up SPF, DKIM and DMARC correctly.
• Use a dedicated sending domain/subdomain.
• Warm up gradually rather than jumping from 5k to 100k sends.
• Suppress chronically unengaged subscribers.
• Separate transactional and marketing mail.
• Monitor Gmail, Outlook and Yahoo reputation separately.
• Don't continually mail your entire 100k list.
• Segment based on engagement.
• Make unsubscribe extremely easy.
• Keep complaint rates exceptionally low.

One subtle point: don't choose an ESP because it advertises a particular "deliverability percentage." No platform can guarantee inbox placement. Your sender reputation and subscriber behavior ultimately matter enormously.

If you tell me what kind of business you have (ecommerce, SaaS, newsletter, coaching/info, marketplace, etc.) and roughly how many emails/SMS you'll send per month, I can narrow this down to the best 2–3 options and compare pricing at 10k, 50k, and 100k subscribers.

viewed product → added to cart → purchased → reordered.

Customer.io's event-first architecture makes it particularly attractive for product-led businesses. productgrowth.in codedrips.com

If you're already approaching enterprise scale

Braze becomes more interesting.

Braze is built for sophisticated cross-channel orchestration—email, SMS, push, in-app, etc.—and is a better long-term fit if you expect hundreds of thousands to millions of users, multiple messaging channels, and a dedicated marketing/engineering team. The tradeoff is substantially more complexity and cost.

What I'd actually do

If I were starting with 5k subscribers and planning for 100k, I'd avoid optimizing around the cheapest provider.

If your goal is a few thousand → 100k+ subscribers while keeping deliverability strong, I’d shortlist Klaviyo, Customer.io, and Braze—but the best choice depends heavily on whether you’re ecommerce, SaaS, or a publisher/community.

My ranking

🥇 My default pick: Klaviyo

klaviyo.com is probably the safest choice if email + SMS are both important and you're likely to grow toward 100k subscribers.

Its deliverability infrastructure is mature, and it supports branded sending domains, dedicated click tracking, shared IPs, and dedicated IPs. Klaviyo currently recommends a branded sending domain once you're above 5,000 profiles, while dedicated IPs generally become relevant at much higher sending volumes. Klaviyo Help Center Klaviyo Academy

The important distinction: 100k subscribers doesn't automatically mean you need a dedicated IP. If you have 100k people but only email them occasionally, a well-managed shared IP can actually be preferable. Klaviyo specifically recommends dedicated IPs for consistently sending around 1M+ emails/month. Klaviyo Help Center

For SMS, Klaviyo also has a clear scaling path: toll-free numbers work for ordinary volumes, while dedicated short codes are particularly useful when you're sending to 100k+ SMS recipients per month because of their much higher throughput. Klaviyo

🥈 Customer.io if you're SaaS/product-led

customer.io would be my pick over Klaviyo if your subscribers are really users of a product, rather than shoppers.

It's particularly strong when you want logic such as:

User signed up → didn't activate → viewed feature X → received email → clicked but didn't convert → send SMS → stop campaign after conversion.

Customer.io supports both email and SMS, and its infrastructure gives you a lot of control as volume grows. It has shared IP pools, dedicated IPs, and a separate transactional IP pool. Dedicated IPs require at least 50,000 emails/week, which is a reasonable threshold for an organization approaching substantial scale. Customer.io Customer.io

A particularly nice feature is that Customer.io can also use your own email delivery provider/SMTP infrastructure if you eventually want even more control over sending. Customer.io

🥉 Braze if you're building an enterprise-scale operation

braze.com is the one I'd consider if you expect 100k to eventually become millions and want a sophisticated omnichannel customer-engagement platform.

But I'd generally not start with Braze at only a few thousand subscribers unless you already have an enterprise-level need and budget. It's more infrastructure than most companies need at that stage.

The bigger deliverability question

The platform matters, but your sending practices matter more.

For your particular growth path, I'd set things up like this:

Now — 2k–10k

• Use a reputable shared IP.
• Authenticate your domain with SPF/DKIM/DMARC.
• Use a branded sending domain.
• Keep marketing and transactional email logically separated.
• Suppress hard bounces and chronically unengaged users.
• Don't suddenly blast your entire imported list.

10k–100k

• Maintain aggressive engagement-based segmentation.
• Gradually ramp volume when substantially increasing sends.
• Monitor Gmail/Yahoo/Microsoft reputation.
• Consider dedicated click tracking.
• Keep SMS consent and opt-outs extremely clean.
• Don't chase volume at the expense of engagement.

100k+

• Look at your actual monthly email volume, not just subscriber count.
• If you're consistently sending ~1M+ emails/month, evaluate a dedicated IP with Klaviyo. Klaviyo Help Center
• If you're sending >50k emails/week and want dedicated infrastructure, Customer.io becomes particularly interesting. Customer.io
• Separate transactional from marketing traffic so a promotional campaign can't damage password-reset/order-confirmation delivery.

And don't assume a dedicated IP is inherently better. Both Klaviyo and Customer.io emphasize that reputation, authentication, engagement, and sending behavior are fundamental; dedicated IPs only make sense once your volume and consistency justify them. Klaviyo Help Center Customer.io

Bottom line

For ecommerce/DTC: → Klaviyo

For SaaS/app/product lifecycle: → Customer.io

For enterprise/multi-million-user omnichannel: → Braze

If I had to choose one without knowing anything else: Klaviyo. It's the best combination of email + SMS + automation + deliverability + a relatively easy path from 5k to 100k+.

If you tell me what kind of business you have (ecommerce, SaaS, newsletter/media, etc.) and roughly how many emails/SMS messages you expect to send per month, I can narrow this down to the best 2–3 and compare pricing at 10k, 50k, and 100k subscribers.

🥉 Braze if you're building an enterprise-scale operation

If your goal is a few thousand → 100k subscribers while keeping deliverability strong, my default pick is Klaviyo—especially if this is ecommerce/DTC. It combines email + SMS, strong segmentation/automation, and a mature deliverability stack without forcing you into an enterprise platform too early. Current 2026 comparisons also put Klaviyo ahead for ecommerce, while Braze is better suited to app-first/enterprise lifecycle programs. Maestra Klaviyo

My ranking

🥇 I'd choose Klaviyo if you're ecommerce

The important thing is that 100k subscribers itself isn't the difficult part. Deliverability depends much more on list quality, engagement, authentication, sending patterns, and reputation than the nominal size of your database. Klaviyo explicitly monitors bounces, complaints, unsubscribes and engagement as deliverability signals. Klaviyo Help Center

Klaviyo is particularly attractive because email and SMS can operate from the same customer profiles and automation system. Its SMS product also handles consent separately from email, which is important because SMS compliance is stricter. Klaviyo Help Center

klaviyo.com

🥈 Customer.io if you're SaaS

I'd pick Customer.io over Klaviyo if you're building around product events rather than purchases—for example:

signup → activation → trial → usage → upgrade → retention

It's designed around event-driven lifecycle messaging and gives you more flexibility around product/customer data.

🥇 Braze if you expect to become enterprise

If you're building a consumer app and expect email + SMS + push + in-app + sophisticated real-time journeys, I'd seriously consider Braze from the beginning. Its current offering is explicitly designed for high-scale, cross-channel programs and includes dedicated deliverability services and IP-warming support. Braze

braze.com

The downside is cost and implementation complexity. For a company going from 3k → 100k, Braze can be unnecessary overhead unless you already have the engineering/marketing infrastructure to exploit it.

What I'd do in your position

Ecommerce: Klaviyo
SaaS/app: Customer.io
Large consumer app / enterprise: Braze
Simple newsletter/publisher: I'd consider a different newsletter-focused platform rather than paying for a full lifecycle stack.

One important caveat: don't choose based on advertised "99%+ deliverability." A platform can provide excellent infrastructure, but it can't rescue a poorly managed list. Set up SPF/DKIM/DMARC, warm sending gradually, suppress chronically unengaged users, use double/confirmed opt-in where appropriate, and keep complaint/bounce rates low.

If you tell me what kind of business this is (ecommerce, SaaS, newsletter, etc.) and roughly how many emails/SMS you'll send per month, I can narrow this to the best 2–3 options and compare their actual cost at 10k, 50k, and 100k subscribers.

For scaling from a few thousand → ~100k subscribers while protecting deliverability, the “best” platform depends heavily on whether you are an ecommerce brand, SaaS/app, creator/media business, or enterprise. There is no single winner, but these are the strongest choices:

Product Growth Braze## My shortlist by use case

If you run ecommerce (Shopify, WooCommerce, subscriptions)

Pick: Klaviyo

This is usually the safest path from 5k → 100k subscribers because:

• email and SMS are unified
• purchase behavior drives segmentation
• abandoned cart, browse abandonment, post-purchase, win-back, and VIP flows are mature
• marketers can operate it without engineering support

Klaviyo is widely positioned around ecommerce data, segmentation, and email/SMS workflows. Klaviyo

If you run SaaS, an app, or a membership product

Pick: Customer.io

Better if your messaging depends on events like:

• signed up but didn't activate
• used feature X
• trial expires in 3 days
• usage dropped
• renewal risk

Customer.io is built around behavioral/event-driven journeys rather than just subscriber lists. Customer.io

If you expect millions of users eventually

Pick: Braze

Braze is where many large consumer apps land because it handles:

• email
• SMS
• push
• in-app messaging
• complex lifecycle journeys

It is designed for large-scale customer engagement, but usually requires more budget and operational maturity. Braze

Deliverability matters more than the logo

At 100k subscribers, the platform is only half the equation. The biggest deliverability factors are:

1. Authenticate your domain

• SPF
• DKIM
• DMARC
• Warm up sending gradually

• Do not import 100k contacts and blast immediately from a new sender/domain.
• Protect engagement rates

• Regularly suppress: - never-openers
• invalid emails
• inactive subscribers
• Separate transactional and marketing reputation

• Password resets, receipts, and alerts should not share the same sending infrastructure as promotions.
• Use segmentation

• A targeted 20k send usually performs better than a generic 100k send.

My practical recommendation

For most companies going from 5k → 100k subscribers:

• Ecommerce: Klaviyo
• SaaS/app: Customer.io
• Enterprise consumer product: Braze
• Newsletter/media: consider Kit or a newsletter-focused ESP plus a dedicated transactional provider

If you tell me:

1. ecommerce vs SaaS vs newsletter,
2. whether SMS is a core channel or just backup, and
3. your expected monthly send volume,

I can narrow this to a specific stack (including whether you should use a dedicated SMTP provider like Amazon Web Services SES, SendGrid, or keep everything inside the ESP).

If your goal is a few thousand → ~100,000 subscribers while keeping email + SMS deliverability strong, my default pick is Klaviyo—especially if you're ecommerce/DTC.

My ranking

Why I'd choose Klaviyo

Klaviyo has a particularly strong combination of email + SMS + segmentation + automation + deliverability controls in one system. It explicitly supports high-volume SMS infrastructure and provides centralized email/SMS deliverability monitoring. Klaviyo Klaviyo Help Center

For email, the important part isn't simply "Klaviyo delivers well." Your sending reputation ultimately matters more than the vendor. Klaviyo recommends using a branded sending domain, DMARC, proper domain alignment, list hygiene, gradual ramping, and engagement-based sending. Klaviyo Help Center Klaviyo Help Center

At 100k subscribers, those practices become much more important than the difference between two reputable ESPs.

One important distinction: 100k subscribers ≠ 100k SMS subscribers

If you're talking about 100k email subscribers, Klaviyo is very comfortable at that scale.

If you're talking about 100k SMS subscribers, I'd pay particular attention to SMS throughput. Klaviyo says short codes are worth considering once you reach ~30k SMS subscribers, and are particularly useful for high-volume sends. Klaviyo Help Center

So I'd structure it roughly like:

Email:
Your domain → Klaviyo → Gmail/Yahoo/Outlook

SMS:
Klaviyo → appropriate toll-free/short-code infrastructure → carriers

And I'd keep your marketing and transactional email streams separated as you grow.

When I'd pick Customer.io instead

If you're SaaS rather than ecommerce, I'd seriously consider Customer.io.

Customer.io is particularly attractive when your messaging depends on product behavior—e.g. user created account → didn't complete setup → used feature X → became inactive → upgrade prompt. Its current deliverability documentation also emphasizes gradual ramping and carrying engagement data over when migrating providers. Customer.io

So my simple decision rule is:

• Shopify/ecommerce/DTC → Klaviyo
• SaaS/product-led → Customer.io
• Large enterprise with very sophisticated orchestration → Braze
• SMS is overwhelmingly more important than email → Attentive
• Need CRM + marketing platform more than best-in-class messaging → HubSpot

The bigger deliverability issue

I wouldn't choose a platform based on claims like "99% deliverability." Nobody can guarantee inbox placement, because Gmail, Yahoo, Microsoft, etc. make the final filtering decisions. Klaviyo itself notes that delivery to the recipient's server doesn't guarantee placement in the primary inbox. Klaviyo Help Center

If you're migrating from another ESP, don't dump all 100k contacts into the new platform and blast them. Warm the new sending infrastructure gradually, starting with your most engaged users. Customer.io and Klaviyo both explicitly recommend this approach. Customer.io Klaviyo Help Center

Bottom line: I'd choose Klaviyo for most businesses going from 3k → 100k. The platform is mature enough for that scale without forcing you into enterprise complexity, and its email/SMS/deliverability tooling is unusually well integrated.

If you tell me (1) ecommerce vs SaaS, (2) roughly how many emails/SMS you'll send per month, and (3) your current platform, I can narrow it down to the best 2–3 options and explain the likely cost at 10k, 50k, and 100k subscribers.

If you're talking about 100k email subscribers, Klaviyo is very comfortable at that scale.

If you're talking about 100k SMS subscribers, I'd pay particular attention to SMS throughput. Klaviyo says short codes are worth considering once you reach ~30k SMS subscribers, and are particularly useful for high-volume sends. Klaviyo Help Center

So I'd structure it roughly like:

Email:
Your domain → Klaviyo → Gmail/Yahoo/Outlook

SMS:
Klaviyo → appropriate toll-free/short-code infrastructure → carriers

And I'd keep your marketing and transactional email streams separated as you grow.

When I'd pick Customer.io instead

If you're SaaS rather than ecommerce, I'd seriously consider Customer.io.

Customer.io is particularly attractive when your messaging depends on product behavior—e.g. user created account → didn't complete setup → used feature X → became inactive → upgrade prompt. Its current deliverability documentation also emphasizes gradual ramping and carrying engagement data over when migrating providers.

So my simple decision rule is:

If your goal is a few thousand → 100k+ subscribers while keeping deliverability strong, I’d shortlist Klaviyo, Customer.io, and Attentive. The best choice depends heavily on whether you’re ecommerce/DTC or a SaaS/content business.

My ranking

1. Klaviyo — my default recommendation

If you're building a consumer brand, newsletter + commerce business, or DTC operation, I'd probably choose Klaviyo.

Its big advantage is that email and SMS live around the same customer profile, so you can coordinate things like:

Email → no engagement → SMS → purchase → suppress both → post-purchase email

Klaviyo also has detailed deliverability reporting and supports enterprise-scale processing/sending. Klaviyo

The important caveat: Klaviyo doesn't magically guarantee inbox placement. Your list hygiene, authentication, engagement, sending patterns, content, and domain reputation matter enormously.

Klaviyo's current pricing also moved mobile messaging to per-message pricing in July 2026, so I'd model your expected SMS volume rather than comparing only subscriber counts. Klaviyo Help Center

klaviyo.com

2. Customer.io — best if you're sophisticated about lifecycle

I'd seriously consider Customer.io if this isn't primarily ecommerce.

It's particularly good when your messaging depends on events and user behavior:

• signed up
• completed onboarding
• used feature X
• hasn't logged in for 14 days
• reached a usage threshold
• abandoned a workflow
• converted from trial → paid

It supports email and SMS, with SMS sent through Twilio, and its current plans are explicitly designed to scale profile and message volume. Premium/Enterprise also offer managed deliverability capabilities. Customer.io Customer.io Customer.io

At 100k subscribers, that's a compelling architecture if you're operating more like a CRM/lifecycle marketing team than a traditional newsletter operation.

3. Attentive — best if SMS is central

I'd choose Attentive over Klaviyo if your business is SMS-first and you're willing to pay for a more hands-on enterprise program.

Attentive positions itself around purpose-built deliverability infrastructure, multiple sending partners, carrier relationships, compliance, and high-volume reliability. Attentive

The tradeoff is cost and complexity. At 100k contacts, you're entering territory where Attentive's higher-touch model can make sense, particularly for a large DTC operation.

What I'd actually do

If you told me only:

"We're at 3–10k subscribers now and expect to reach 100k, and email + SMS deliverability is a top priority."

I'd pick:

Klaviyo → unless you're SaaS/product-led → Customer.io.

And I would not switch platforms merely because another vendor claims a slightly higher deliverability rate. Deliverability is primarily an operational discipline, not a checkbox on a vendor comparison.

At 100k, I'd prioritize:

1. Dedicated sending domain/subdomain strategy
2. SPF, DKIM and DMARC configured correctly
3. Gradual domain/IP warm-up where appropriate
4. Aggressive suppression of chronically unengaged contacts
5. Separate transactional vs. marketing mail
6. Double opt-in or very high-quality acquisition sources
7. Automated bounce/complaint handling
8. Google Postmaster Tools monitoring
9. SMS consent/compliance and carrier filtering
10. A platform with good deliverability diagnostics, not merely a claimed delivery percentage

One important distinction: "delivered" is not the same as "inbox." A platform can report 99%+ delivery while some of those messages land in spam.

If you tell me what kind of business this is (ecommerce, SaaS, newsletter/media, marketplace, etc.) and roughly how many emails/SMS you'll send per month, I can narrow this down to the best 2–3 choices and estimate what 100k subscribers will actually cost.

The tradeoff is cost and complexity. At 100k contacts, you're entering territory where Attentive's higher-touch model can make sense, particularly for a large DTC operation.

What I'd actually do

If you told me only:

"We're at 3–10k subscribers now and expect to reach 100k, and email + SMS deliverability is a top priority."

I'd pick:

Klaviyo → unless you're SaaS/product-led → Customer.io.

It's particularly good when your messaging depends on events and user behavior:

If your goal is a few thousand → ~100,000 subscribers without sacrificing email/SMS deliverability, my default recommendation is Klaviyo—especially for a DTC/e-commerce business.

My ranking

🥇 Klaviyo — best overall

I'd choose Klaviyo if you're starting around 2–10k subscribers and expect to reach 100k.

Its biggest advantage isn't simply raw sending capacity. It's that email + SMS + segmentation + automation + deliverability tooling are integrated, so you don't have to rebuild your stack as you grow. Klaviyo specifically provides separate email and SMS deliverability monitoring, including sender-reputation metrics and diagnostics. Klaviyo Help Center Klaviyo Help Center

It also supports high-volume SMS infrastructure: Klaviyo says toll-free numbers can reach 25 messages/second, while dedicated short codes can scale substantially higher and are appropriate when you're sending to >100k people/month. Klaviyo

klaviyo.com

Why I'd pick it: excellent balance of deliverability, ease of use, segmentation, flows, analytics, email and SMS.

🥈 Braze — best if you're building a serious lifecycle engine

If you're not primarily e-commerce and expect to eventually have hundreds of thousands or millions of users, I'd seriously consider Braze.

Braze has particularly strong infrastructure for very large-scale, event-driven messaging. It offers email, SMS, push, in-app, RCS and other channels in one system, and provides dedicated deliverability services including IP warming, monitoring and deliverability specialists. Braze Braze

braze.com

The downside is cost, implementation complexity and technical overhead. At 100k subscribers, you may be paying for capabilities you don't yet need.

🥉 Attentive — strongest alternative if SMS is central

Attentive is particularly compelling if your strategy is heavily SMS + email + commerce. Its current positioning emphasizes email, SMS and push together, with dedicated deliverability and migration support. Attentive

attentive.com

I'd put it ahead of Klaviyo if SMS is the primary revenue channel and you have a sophisticated commerce operation.

The important part: the platform isn't what ultimately determines deliverability

Going from 5k → 100k doesn't inherently create a deliverability problem. Bad list acquisition and sending behavior do.

I'd prioritize:

1. A dedicated sending domain with SPF, DKIM and DMARC configured correctly.
2. Gradual volume ramping rather than suddenly sending to 100k people.
3. Aggressive suppression of chronically unengaged subscribers.
4. Double opt-in where appropriate.
5. Separate transactional and marketing sending infrastructure.
6. Strong segmentation so you're not blasting your entire database unnecessarily.
7. Monitoring Gmail/Yahoo/Microsoft reputation and spam complaints.
8. For SMS, proper A2P/10DLC or short-code registration and conservative sending practices.

Klaviyo's current deliverability guidance emphasizes infrastructure, warming, monitoring, authentication and list hygiene; it also recommends sending only to consented profiles and cleaning unengaged/invalid addresses. Klaviyo Help Center Klaviyo Help Center

And for SMS, carrier filtering is a major consideration, independent of the marketing platform. Klaviyo Help Center Klaviyo Help Center

My practical recommendation

If you told me:

"We're at 3,000 subscribers today and want to be at 100,000 within 1–2 years."

I'd start with Klaviyo, build the data model and lifecycle flows correctly from day one, and stay there through 100k unless your requirements become substantially more enterprise/technical.

If you told me:

"We're building a product with millions of users, complex behavioral events, mobile push, in-app messaging and sophisticated journeys."

I'd start with Braze instead.

For most businesses, the sweet spot is Klaviyo. The bigger mistake would be choosing an enterprise platform prematurely—or choosing a cheap/simple ESP now and having to migrate when you're already at 100k.

The downside is cost, implementation complexity and technical overhead. At 100k subscribers, you may be paying for capabilities you don't yet need.

🥉 Attentive — strongest alternative if SMS is central

Attentive is particularly compelling if your strategy is heavily SMS + email + commerce. Its current positioning emphasizes email, SMS and push together, with dedicated deliverability and migration support. attentive.com

attentive.com

I'd put it ahead of Klaviyo if SMS is the primary revenue channel and you have a sophisticated commerce operation.

The important part: the platform isn't what ultimately determines deliverability

Going from 5k → 100k doesn't inherently create a deliverability problem. Bad list acquisition and sending behavior do.

If you're scaling from a few thousand to ~100,000 subscribers, and your priorities are email + SMS in one platform, strong deliverability, and not having to migrate again, my shortlist would be:

My pick: Klaviyo

For most businesses, I'd choose Klaviyo if the primary goal is growing a subscriber list while keeping email and SMS tightly integrated.

A major reason is that Klaviyo puts considerable emphasis on sender reputation: it recommends a branded sending domain, supports dedicated infrastructure for sufficiently high-volume senders, and automatically suppresses problematic addresses to protect reputation. academy.klaviyo.com

For SMS, Klaviyo supports toll-free numbers and dedicated short codes; it notes that short codes are particularly appropriate when you're sending SMS to 100,000+ people per month. www.klaviyo.com

Klaviyo

When I'd choose Customer.io instead

If you're SaaS, an app, membership business, marketplace, or product-led company, I'd seriously consider Customer.io over Klaviyo.

Customer.io is particularly strong when you want event-driven journeys such as:

signup → onboarding → activation → usage milestone → upsell → churn prevention

Its SMS architecture is also unusually flexible: you can connect Twilio, Sinch, Infobip, or Vonage, allowing you to choose different SMS providers for different regions. docs.customer.io

It also offers shared and dedicated email IP infrastructure, including a separate transactional IP pool with stricter bounce/spam thresholds. docs.customer.io

Customer.io

What I would not optimize for

Don't assume that buying a dedicated IP automatically means better deliverability. At 10k–100k subscribers, your domain reputation, engagement, authentication, complaint rate, bounce rate, and sending behavior are usually more important. Klaviyo, for example, recommends dedicated IPs primarily for customers consistently sending 1M+ emails/month. academy.klaviyo.comhelp.klaviyo.com

Also, migrating platforms at 50k–100k subscribers can temporarily hurt deliverability because mailbox providers see a change in your sending infrastructure. Customer.io explicitly recommends gradually warming a new sender and starting with highly engaged recipients. docs.customer.io

The setup I'd use

If I were building this today for 100k subscribers:

Klaviyo

• branded sending domain
• SPF/DKIM/DMARC correctly configured
• separate transactional vs marketing sending where appropriate
• aggressive suppression of unengaged/bounced users
• SMS with the appropriate local sender/number
• automated re-engagement → suppression flow
• gradual volume increases rather than giant blasts

One important distinction: 100k subscribers doesn't necessarily mean 100k active recipients. A clean, highly engaged 60k-person list can produce substantially better deliverability than a 100k list full of inactive addresses.

Bottom line:

• 🛒 E-commerce/DTC → Klaviyo
• 🚀 SaaS/app/product → Customer.io
• 🏢 Large enterprise with very complex orchestration → Braze
• 🔄 CRM is the center of your operation → HubSpot

If you tell me what kind of business you have, roughly how many emails/SMS you'll send per month, and whether the 100k subscribers are mostly US or international, I can narrow this down to the best platform and estimate what it will cost at 5k → 25k → 50k → 100k subscribers.

For scaling from a few thousand → 100k+ subscribers while keeping deliverability high, the platform matters, but your sending practices, domain reputation, list quality, segmentation, and authentication matter just as much. At 100k subscribers, a mediocre setup can destroy inbox placement even on a premium ESP.

The best choice depends on your business model:

www.klaviyo.comaiproductivity.ai

My default picks

If you are ecommerce (Shopify, DTC, subscriptions):
→ Klaviyo is usually the best choice. It combines email + SMS, strong behavioral segmentation, and ecommerce attribution. It is widely used by brands scaling into six-figure subscriber lists. www.klaviyo.com

If you are SaaS/community/content with user behavior data:
→ Customer.io is often a better fit because you can trigger campaigns from product events (signup, usage, inactivity, milestones). www.klaviyo.comaiproductivity.ai

If you expect millions of users, mobile apps, or complex omnichannel journeys:
→ Braze is the enterprise choice. It is built for sophisticated lifecycle orchestration across email, SMS, push, and in-app channels. www.rackwave.io

Deliverability checklist for reaching 100k subscribers

Regardless of platform:

1. Set up authentication correctly

You should have:

• SPF
• DKIM
• DMARC (ideally with a monitoring phase before enforcement)
• Custom sending domain

This is more important than the ESP brand.

2. Do not migrate 100k subscribers overnight

A good ramp:

• Week 1: most engaged 5–10%
• Week 2: expand to recent engagers
• Weeks 3–6: gradually include older subscribers

Mailbox providers care about engagement history.

3. Segment aggressively

At 100k, avoid blasting everyone.

Typical segments:

• Active in last 30 days
• Active in last 90 days
• Purchased but inactive
• High-value customers
• New subscribers
• Dormant users

4. Protect your sender reputation

Avoid:

• Purchased lists
• Sudden volume spikes
• Sending to years-old inactive contacts
• High complaint rates
• Excessive promotional frequency

5. Consider separating email types

Many larger companies separate:

• Marketing email → ESP
• Transactional email → dedicated transactional provider

For example:

• Marketing: Klaviyo / Customer.io / Braze
• Transactional: Amazon Simple Email Service, Postmark, etc.

My recommendation by scenario

1. Ecommerce brand growing from 5k → 100k

Klaviyo + SMS + strong segmentation

2. Newsletter/media business

Consider Beehiiv or ConvertKit depending on monetization needs

3. SaaS/product-led company

Customer.io

4. Venture-backed consumer app expecting millions

Braze

5. Lowest cost while still scaling

A combination of a lower-cost ESP + dedicated transactional infrastructure can work, but you need more operational expertise.

If you tell me your business type (ecommerce, SaaS, newsletter, community, app), monthly email volume, SMS importance, and current platform, I can narrow this to a specific recommendation.

For scaling from a few thousand subscribers to ~100k subscribers while protecting deliverability, the “best” platform depends heavily on whether you are ecommerce, SaaS, media/community, or app-based. At 100k, the platform matters—but your sending practices, domain setup, list hygiene, and engagement matter just as much. help.klaviyo.comsupport.iterable.com

My shortlist:

branch8.com

My recommendation by scenario

1. Ecommerce brand (Shopify, DTC, subscriptions)
→ Klaviyo + possibly Attentive/Postscript for SMS-heavy programs

This is probably the safest choice from 5k → 100k subscribers. It gives you:

• welcome flows
• abandoned cart
• post-purchase
• replenishment
• segmentation by behavior
• email + SMS in one customer profile

Klaviyo is particularly strong when your revenue depends on owned-channel retention. www.klaviyo.com

2. SaaS / app / marketplace
→ Braze or Customer.io

You usually care less about campaigns and more about:

• activation sequences
• onboarding
• lifecycle events
• churn prevention
• product usage triggers

Braze is built for this scale, while Customer.io is often a better fit for startups that want flexibility without enterprise pricing. www.klaviyo.com

3. Large content/community business
→ Iterable

If you have many segments, multiple channels, and complicated journeys, Iterable is often a better fit than ecommerce-first tools. mailfloss.com

Deliverability checklist for hitting 100k

Regardless of platform:

1. Use a dedicated sending domain
   • Example: mail.yourbrand.com
   • Keep marketing reputation separate from your main corporate email.

2. Set up authentication
   • SPF
   • DKIM
   • DMARC

3. Warm up gradually
   • Do not import 100k subscribers and blast them immediately.
   • Start with your most engaged users.

4. Keep your list clean
   • Remove invalid addresses.
   • Suppress long-term inactive subscribers.
   • Avoid purchased lists.

5. Separate transactional and marketing email
   • Password resets, receipts, and alerts should not share reputation with promotions.

These factors are major drivers of inbox placement, regardless of ESP. support.iterable.com

If I had to pick one default answer:

• DTC/ecommerce: Klaviyo
• SaaS/product-led: Customer.io or Braze
• Enterprise multi-channel: Iterable or Braze

A key question: are these 100k subscribers primarily customers buying products, app users, or newsletter/community subscribers? That changes the recommendation quite a bit.

For scaling from a few thousand → 100k+ subscribers while protecting deliverability, the “best” platform depends heavily on whether you are ecommerce, SaaS/app, creator/media, or B2B. There is no single winner, but these are the strongest choices:

www.getvero.comwww.hesper.io

My shortlist by scenario

1. Ecommerce brand (products, Shopify, subscriptions)
→ Pick Klaviyo.

Why:

• Excellent purchase/browse behavior segmentation
• Strong email + SMS together
• Good prebuilt flows (welcome, abandoned cart, post-purchase, winback)
• Designed around revenue attribution

The main risk at 100k contacts is cost and database hygiene, not sending capacity. www.hesper.io

2. SaaS/app/community product
→ Pick Customer.io.

Why:

• Built around events (“signed up,” “completed lesson,” “used feature,” “inactive 14 days”)
• Better than ecommerce-first tools for lifecycle messaging
• Handles email, SMS, push, and in-app journeys

It is often the sweet spot before moving into expensive enterprise platforms. www.getvero.com

3. You expect 1M+ users, multiple channels, global markets
→ Pick Braze or Iterable.

These are closer to enterprise customer engagement systems:

• complex journeys
• large event streams
• push/in-app/SMS/email orchestration
• stronger governance

The tradeoff is cost and implementation complexity. www.getvero.com

Deliverability matters more than the vendor

At 100k subscribers, the platform is only half the equation. The biggest factors are:

Authentication

Set up:

• SPF
• DKIM
• DMARC (eventually with enforcement)
• branded sending domains

List quality

Avoid:

• purchased lists
• old inactive contacts
• sending the same message to everyone

Mailbox providers care heavily about engagement signals.

Sending strategy

Do:

• warm up gradually
• segment engaged users
• sunset inactive subscribers
• separate transactional and marketing traffic when appropriate

Infrastructure

For larger senders:

• consider dedicated IPs only when volume and consistency justify it
• monitor inbox placement, not just “delivered”

If I were choosing today:

• Shopify/ecommerce: Klaviyo
• SaaS/product-led: Customer.io
• Fast-growing company that may become enterprise: Customer.io first, then Braze/Iterable when complexity demands it
• Enterprise from day one: Braze

For most companies growing from 5k → 100k subscribers, I would avoid jumping straight to an enterprise platform. A well-configured Klaviyo or Customer.io setup with excellent list hygiene will usually outperform a more expensive tool with poor sending practices. getoden.com

If you tell me your business type (ecommerce, SaaS, newsletter/media, B2B, etc.), monthly email volume, and whether SMS is a major channel, I can narrow this to a specific recommendation.

For scaling from a few thousand → 100k+ subscribers while keeping deliverability high, the “best” platform depends more on your business model than list size. At 100k, almost any major ESP can send the volume; the difference is whether it helps you maintain sender reputation, segmentation, engagement, and automated journeys. Deliverability is driven heavily by your sending practices (authentication, list hygiene, engagement, complaint rates), not just the vendor. customer.io

My shortlist:

www.braze.comwww.klaviyo.com

My practical recommendations

If you are ecommerce (Shopify, DTC, subscriptions):
→ Choose Klaviyo. It is probably the safest path from 5k to 100k because email and SMS are tightly connected to customer behavior: abandoned cart, post-purchase, replenishment, VIP, winback, etc. www.klaviyo.com

If you are SaaS or an app:
→ Choose Customer.io or Braze. Your growth will depend on event-triggered messaging (“user did X, send Y”), not just newsletters. aiproductivity.ai

If you expect to become a major consumer platform (millions of users):
→ Start evaluating Braze earlier. It is built for complex customer journeys and large-scale orchestration. www.braze.com

Deliverability checklist (more important than the ESP)

To protect inbox placement as you grow:

1. Use a dedicated sending domain
   • Example: mail.yourbrand.com
   • Keep marketing reputation separate from transactional email.

2. Set up authentication
   • SPF
   • DKIM
   • DMARC (eventually with enforcement)

3. Warm gradually
   • Do not import 100k subscribers and blast them on day one.
   • Start with your most engaged users.

4. Avoid sending to inactive people
   • A 100k list with 20k engaged subscribers is often healthier than a 300k list full of dead addresses.

5. Segment aggressively
   • New subscribers
   • Active customers
   • High-value customers
   • Dormant users
   • Recent purchasers

If I were choosing today:

• Ecommerce → Klaviyo
• SaaS/product-led → Customer.io
• Enterprise consumer app → Braze
• Newsletter/media business → consider a newsletter-focused platform plus a dedicated transactional provider

The biggest mistake is choosing based on “can it handle 100k contacts?” Almost all can. The real question is whether it helps you send fewer, more relevant messages to the right people as the database grows.

If your goal is a few thousand → ~100k subscribers while keeping email + SMS deliverability strong, my default pick would be Klaviyo.

My ranking

1. 🥇 Klaviyo — best default choice

For a brand that primarily wants email + SMS marketing, I'd choose Klaviyo.

Its deliverability tooling now covers both channels, including account-level email/SMS deliverability reporting. Its email deliverability system tracks things such as bounces, complaints, unsubscribes, engagement and sender reputation. help.klaviyo.com

The important thing is that 100k subscribers itself isn't the hard part. The hard part is maintaining a good sender reputation as you scale. Klaviyo explicitly recommends list hygiene, gradual ramping, authentication and engagement-based sending; its documentation notes that ramping toward 100k volume can take roughly 10 days, with reputation continuing to establish afterward. help.klaviyo.com

I'd pick Klaviyo if:

• You're e-commerce/DTC.
• Email and SMS are your primary channels.
• You want marketers—not engineers—to run campaigns.
• You want strong segmentation and automation.
• You expect to reach 50k–100k+ subscribers without rebuilding your stack.

2. 🥈 Customer.io — best for behavioral/product messaging

I'd choose Customer.io over Klaviyo if you're a SaaS/app/product where messaging depends heavily on things like:

signed up → didn't activate → used feature X → abandoned workflow → upgraded → became inactive

Customer.io is designed around event/behavior-driven journeys and supports email, SMS, push, in-app, etc. Its email infrastructure includes authentication and monitoring for reputation/denylists, and it explicitly recommends gradually ramping sending volume. docs.customer.io

SMS can be handled through Customer.io, with Twilio as the underlying provider in many configurations. docs.customer.io

One caveat: if you're mostly doing straightforward promotional email/SMS, Customer.io can be more machinery than you need.

3. 🥉 Braze — best if you're building for serious scale

Braze is the one I'd consider if you expect your messaging operation to eventually become very sophisticated: email + SMS + push + in-app + RCS + complex personalization and orchestration.

Braze explicitly positions its platform around scaling email without sacrificing deliverability, and it supports SMS/RCS alongside other channels. www.braze.com

The downside is that it's usually overkill at a few thousand subscribers. I'd rather start with Klaviyo or Customer.io unless you already have the organizational/technical complexity that justifies Braze.

What I'd actually do

If you told me:

“We're at 3,000 subscribers today and expect to hit 100,000. We care enormously about inbox placement and want email + SMS.”

I'd go:

E-commerce/DTC → Klaviyo

SaaS/app/product → Customer.io

Large enterprise / highly sophisticated lifecycle → Braze

And I would not choose based primarily on the platform claiming the highest deliverability percentage. Your list quality, consent, engagement, authentication, sending patterns and reputation matter enormously. Klaviyo itself emphasizes that deliverability depends on recipient engagement, sending habits, content and sending infrastructure. help.klaviyo.com

One particularly important point: don't treat email and SMS as one deliverability problem. Email reputation is largely about mailbox providers and sender reputation; SMS is heavily influenced by carrier filtering and messaging practices. help.klaviyo.com

My short answer

If you want one platform and don't have an unusual use case: choose Klaviyo.

If you tell me what kind of business this is (e-commerce, SaaS, newsletter/media, marketplace, etc.) and roughly how many emails/SMS you'll send per month, I can narrow it down to the best 2–3 options and compare pricing at 10k, 50k, and 100k subscribers, including whether I'd use shared or dedicated sending infrastructure.

For scaling from a few thousand → 100k subscribers while protecting deliverability, the “best” platform depends less on list size and more on your business model. At 100k, almost every major platform can technically send; the difference is segmentation, automation, data quality controls, SMS compliance, and deliverability tooling. jakdigitalhub.comwww.klaviyo.com

My shortlist:

www.braze.com

My recommendations by scenario

1. Ecommerce store (Shopify, WooCommerce, DTC)
→ Pick Klaviyo.
It is probably the strongest fit if your goal is:

• abandoned cart flows
• browse abandonment
• post-purchase sequences
• VIP/customer value segmentation
• email + SMS in one system

www.klaviyo.com

2. SaaS product, app, membership, marketplace
→ Pick Customer.io.
You’ll want event-driven messages like:

• “user signed up but never activated”
• “feature not used after 14 days”
• “subscription renewal coming”
• “usage dropped”

Customer.io is built around this style of lifecycle automation. launchly.media

3. Media/newsletter/community business
→ Consider:

• Kit for creator/newsletter workflows
• Customer.io if you need advanced segmentation
• A dedicated sending provider + custom stack if email volume becomes massive

Deliverability: the platform is only half the equation

At 100k subscribers, deliverability usually depends more on your practices than the ESP:

Do:

• Use a dedicated sending domain (e.g., mail.yourdomain.com)
• Set up SPF, DKIM, and DMARC
• Warm up gradually when migrating
• Remove inactive subscribers regularly
• Segment engaged vs. dormant users
• Avoid sudden volume spikes
• Keep complaint rates extremely low

Avoid:

• Buying lists
• Sending to old unengaged contacts
• “Blast everyone” campaigns after months of inactivity

A clean 50k engaged list often outperforms a 200k stale list.

If I were choosing today:

• Ecommerce: Klaviyo
• SaaS/product: Customer.io
• Enterprise app with millions of users: Braze
• Newsletter/media: Kit or Customer.io depending on complexity

For most companies going from 5k → 100k subscribers, I would optimize for segmentation and lifecycle automation first, not just raw sending capacity. The wrong migration at 50k–100k contacts can hurt inbox placement for months. launchly.media

If you tell me your type of business (ecommerce, SaaS, newsletter, nonprofit, etc.), current monthly sends, and whether SMS is a core channel or just a bonus, I can narrow it to 1–2 choices.

If your goal is a few thousand → ~100,000 subscribers while keeping email + SMS deliverability strong, my default recommendation is Klaviyo.

My ranking

🥇 Klaviyo — best overall

I'd choose Klaviyo if you're building a traditional marketing list and want email and SMS under one roof.

Its current deliverability tooling includes account-level email/SMS monitoring, bounce and blocklist information, dedicated sending domains/IP options, authentication guidance, and warming tools. help.klaviyo.comdocs.customer.iowww.braze.com

For SMS specifically, Klaviyo reports 97%+ SMS deliverability and supports higher-throughput options such as dedicated short codes when you're sending at very large volumes. www.klaviyo.com

The bigger advantage is that you can build your segmentation, flows, email and SMS around the same customer profile rather than stitching multiple systems together.

Klaviyo's deliverability documentation

🥈 Customer.io — potentially better for SaaS

I'd pick Customer.io over Klaviyo if your business is SaaS/product-led and your messaging is driven by things like:

• user actions
• product usage
• onboarding stages
• account attributes
• complex behavioral sequences
• transactional + lifecycle communication

Customer.io has particularly good control over sending infrastructure: you can use Customer.io's managed email delivery or your own provider/SMTP, and its deliverability reporting can break performance down by recipient provider. docs.customer.io

It also verifies accounts and signup sources specifically as part of maintaining its sending standards. docs.customer.io

🥉 Braze — excellent, but probably overkill at first

Braze is what I'd investigate if you expect to become a fairly sophisticated lifecycle-marketing organization.

It's extremely strong for multichannel orchestration and large-scale personalization, and Braze has dedicated deliverability expertise and infrastructure. www.braze.com

But for going from 3k → 100k subscribers, I'd generally rather put the money and operational complexity into Klaviyo or Customer.io unless you already know you'll need Braze's enterprise capabilities.

The important part: the platform isn't what guarantees deliverability

At 100k subscribers, your sending practices matter at least as much as your ESP.

I'd set things up from day one with:

1. Dedicated sending domain — e.g. email.yourbrand.com
2. SPF, DKIM and DMARC properly configured
3. Gradual volume ramp rather than suddenly blasting 100k
4. Double opt-in or very high-quality acquisition sources
5. Automatic suppression of hard bounces
6. Aggressive sunset/re-engagement policies for inactive subscribers
7. Separate transactional and marketing streams
8. Consistent sending patterns
9. Engagement-based segmentation
10. SMS consent collected separately from email consent

That last point is especially important: email opt-in does not automatically constitute SMS consent. help.klaviyo.com

What I'd actually choose

E-commerce/DTC: → Klaviyo

SaaS / app / membership product: → Customer.io

Enterprise with a sophisticated lifecycle team: → Braze

If you tell me what kind of business you have, your current subscriber count, approximate monthly email sends, and whether SMS is primarily marketing or transactional, I can narrow this down to the best platform and give you a 3k → 10k → 50k → 100k migration/architecture plan, including when to introduce a dedicated IP and how I'd structure the domains.

If your goal is a few thousand → 100k+ subscribers while maintaining strong email + SMS deliverability, my default recommendation is Klaviyo.

My ranking

🥇 I'd choose Klaviyo for your situation

Klaviyo is particularly attractive because you don't need to migrate again when you go from 5k → 25k → 100k subscribers. It supports both email and SMS, sophisticated segmentation/automation, and infrastructure designed for high-volume sending. Its current guidance recommends branded sending domains, proper authentication, warming, list hygiene, and dedicated infrastructure as volume increases. help.klaviyo.comhelp.klaviyo.com

One important point: 100k subscribers does not automatically mean you need a dedicated IP. Klaviyo currently recommends dedicated IPs primarily for senders consistently exceeding roughly 1 million emails/month; below that, a properly configured shared infrastructure can actually be preferable. help.klaviyo.comhelp.klaviyo.com

For SMS, Klaviyo supports toll-free numbers, 10DLC and dedicated short codes. Its documentation says short codes become particularly relevant when you're sending to 100k+ people per month, because they can provide much higher throughput. www.klaviyo.comwww.braze.comhelp.klaviyo.com

When I'd pick Braze instead

If you're building a large-scale consumer app, SaaS product, marketplace, or enterprise lifecycle program, I'd seriously consider Braze.

Braze is built around sophisticated cross-channel orchestration—email, SMS/MMS, push, in-app, WhatsApp, RCS, etc.—and is designed to scale well beyond 100k users. Its infrastructure provisions capacity based on customer size and projected growth. www.braze.com

The tradeoff is cost and complexity. At only a few thousand subscribers, Braze is usually overkill unless you already have sophisticated product/event data and lifecycle requirements.

The bigger deliverability decision

Don't choose a platform expecting it to guarantee inbox placement. At 100k subscribers, your sending practices become at least as important as the ESP.

I'd set up:

• A dedicated/branded sending domain
• SPF, DKIM and DMARC correctly aligned
• A separate tracking domain
• Gradual warming when migrating
• Double opt-in or very high-quality acquisition
• Automated suppression of chronically unengaged subscribers
• Google Postmaster Tools monitoring
• Separate transactional and marketing streams where appropriate
• SMS consent/10DLC compliance
• Gradual SMS volume increases rather than blasting a newly acquired list

Klaviyo specifically recommends branded sending domains and DMARC, and notes that new infrastructure needs warming/ramping. help.klaviyo.com

Bottom line:
Ecommerce/DTC → Klaviyo.
SaaS/product with complex behavioral journeys → Customer.io or Braze.
Enterprise/highly sophisticated omnichannel → Braze.
Budget-conscious ecommerce → Omnisend.

If you tell me what kind of business you have (ecommerce, SaaS, newsletter/media, etc.), approximate monthly email volume, and whether SMS is mainly promotional or transactional, I can narrow this down to the best platform and the architecture I'd use to get from 5k to 100k without hurting deliverability.

If your goal is a few thousand → ~100k subscribers while keeping email and SMS deliverability strong, my default choice would be Klaviyo—especially for ecommerce/DTC.

My ranking

Why I'd pick Klaviyo

Klaviyo has a particularly good combination of email + SMS + segmentation + automation + deliverability tooling. Its current deliverability tooling covers domain authentication, warming, bounces, reputation, engagement, and dedicated IPs. help.klaviyo.com

For SMS, it also has a dedicated deliverability dashboard and tools for monitoring failures, carrier filtering, consent, and sending reputation. help.klaviyo.com

Most importantly, 100k subscribers isn't remotely an unusual scale for the platform. Klaviyo's infrastructure is designed for high-volume sending, and it supports higher-throughput SMS configurations as lists grow. www.klaviyo.com

Klaviyo's deliverability documentation

When I'd choose something else

Choose Braze if you're building a sophisticated product/app with email + SMS + push + in-app + web messaging and expect to need complex real-time orchestration. Braze is built for considerably larger organizations and supports extensive cross-channel journey orchestration. www.braze.com Its deliverability infrastructure includes IP warming and IP pools; Braze cites examples of customers scaling very large email volumes while maintaining strong deliverability. www.braze.com

Choose Customer.io if you're SaaS/product-led and want highly event-driven messaging. It gives you control over shared vs. dedicated IPs and has explicit deliverability controls around authentication, engagement, complaints, and list hygiene. docs.customer.io

Choose Iterable if you're somewhere between Klaviyo and Braze in terms of organizational complexity. It has strong email deliverability tooling and unified email/SMS/customer data. support.iterable.com

One important caveat

The platform is only part of deliverability. At 100k, your sending practices matter enormously.

I'd set up:

• Dedicated sending domain/subdomain
• SPF + DKIM + DMARC correctly authenticated
• Gradual volume ramp rather than suddenly mailing 100k
• Double opt-in or very strong signup validation
• Automatic suppression of hard bounces and chronically inactive subscribers
• Engagement-based segments
• A sunset policy for genuinely inactive people
• Separate transactional and marketing streams where appropriate
• Consistent sending cadence
• SMS consent captured separately from email consent
• Proper US SMS registration/number configuration
• Careful monitoring of spam complaints, bounces, unsubscribes, and Gmail/Microsoft engagement

This is more important than chasing a platform that claims a marginally better "deliverability rate." Inbox providers ultimately care heavily about your domain/sender reputation and recipient behavior. Klaviyo, Customer.io, Braze, and Iterable all emphasize this. help.klaviyo.com

Bottom line: If you told me, "We're at 3–5k now, expect 100k, and want one platform for email + SMS without having to re-platform," I'd start with Klaviyo. If you're a SaaS/app rather than ecommerce, I'd seriously consider Customer.io; if you already know you'll need enterprise-grade omnichannel orchestration, Braze is the stronger long-term choice.

If your goal is a few thousand → ~100k subscribers while keeping deliverability strong, my default recommendation is Klaviyo—especially if this is ecommerce/DTC.

My pick: Klaviyo

Klaviyo is particularly strong because email and SMS live around the same customer profile, with behavioral segmentation and automated journeys. Current 2026 comparisons continue to put it ahead for ecommerce, while Braze is more compelling for app-first/enterprise organizations. www.klaviyo.comagence-deliver.com

The bigger point, though: the ESP doesn't guarantee deliverability. At 100k subscribers, list quality, engagement, authentication, sending patterns, and reputation matter enormously. Klaviyo's own current guidance, for example, considers bounce rate under 1%, unsubscribe rate under 0.3%, and spam complaints under 0.01% healthy targets. help.klaviyo.com

What I'd choose by business model

Ecommerce / Shopify:
Klaviyo → best balance of scaling, segmentation, email + SMS, ecommerce data, and deliverability.

SaaS / app / product-led business:
Customer.io → better if your campaigns are driven by detailed product events and a custom data model. www.klaviyo.comagence-deliver.com

Mobile app + sophisticated omnichannel lifecycle:
Braze → excellent once you're large enough to justify the additional technical complexity and cost. Braze specifically offers dedicated deliverability services, IP warming, and deliverability specialists for large programs. www.braze.com

Large list but relatively infrequent sends / cost is paramount:
Brevo can make economic sense because its pricing is more send-volume-oriented, although I'd prioritize Klaviyo/Braze/Customer.io when deliverability and lifecycle sophistication are the primary objectives. www.inboxeagle.com

One important recommendation: don't wait until 100k to think about deliverability. Set up SPF, DKIM and DMARC correctly, maintain a clean opt-in list, suppress chronically unengaged users, segment by engagement, and ramp sending volume gradually. A 100k-person list with poor engagement can perform much worse than a 20k highly engaged list.

If you tell me what kind of business this is (ecommerce, SaaS, newsletter/media, community, etc.) and roughly how many emails/SMS you'll send per month, I can narrow this to the best 1–2 platforms and compare their likely cost at 10k, 50k, and 100k subscribers.