ChatGPT answers, unedited

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.

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 → ~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.

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.