ChatGPT answers, unedited

If your core requirement is “a user does X on my website → send them a personalized email/SMS sequence”, I’d shortlist these:

My #1: Customer.io

customer.io

For what you described, Customer.io is probably the first one I'd evaluate.

You can send events from your site/product such as:

• signed_up
• viewed_pricing
• started_trial
• invited_teammate
• created_project
• used_feature
• abandoned_checkout
• subscription_expiring

…and use those events to enter users into multi-step journeys. Customer.io supports real-time event triggers, branching logic, delays, personalization, and email/SMS in the same workflow. Customer.io Customer.io Customer.io

For example:

User views pricing →

1. Wait 30 minutes
2. If they haven't signed up → personalized email
3. Wait 1 day
4. If still inactive → SMS
5. If they sign up → stop the sequence
6. If they return to pricing → send a different message
7. If they become highly engaged → notify sales

That's exactly the sort of workflow Customer.io is designed around.

When I'd choose Braze instead

braze.com

Choose Braze if you're building a substantial lifecycle/engagement operation and expect sophisticated cross-channel personalization.

Braze can react to real-time behavior and coordinate email, SMS/RCS, push, in-app, web messaging, and other channels. It also has more advanced experimentation and AI-driven decisioning. Braze Braze Braze

I'd generally think:

Customer.io → startup / SaaS / product-led growth

Braze → larger company / sophisticated consumer engagement

When I'd choose Klaviyo

klaviyo.com

If this is an ecommerce site, I'd strongly consider Klaviyo instead. It's particularly good when your behavioral data revolves around products, carts, purchases, browsing, customer profiles, and ecommerce revenue.

What I'd pick by scenario

• SaaS / web app: Customer.io
• Startup doing lifecycle marketing: Customer.io
• Ecommerce / Shopify: Klaviyo
• Large consumer app: Braze
• Enterprise omnichannel: Braze or Iterable
• CRM + marketing automation: ActiveCampaign
• Very sophisticated, high-volume behavioral messaging: Braze

One important distinction: don't optimize primarily for “email marketing.” What you want is a customer engagement/event-triggering platform that treats your site's behavioral events as first-class data. That's what makes sequences like viewed_pricing → didn't signup → returned → purchased practical rather than trying to bolt automation onto a traditional newsletter platform.

If you tell me what your site does, roughly how many users you have, and whether you're B2B SaaS/ecommerce/B2C, I can narrow this to the best 2–3 options and compare pricing, implementation effort, event tracking, and SMS costs.

I'd generally think:

Customer.io → startup / SaaS / product-led growth

Braze → larger company / sophisticated consumer engagement

When I'd choose Klaviyo

For behavior-triggered personalized email + SMS, I’d shortlist these:

🥇 My default recommendation: Customer.io

If you're building something like:

User visits pricing page → doesn't sign up → 2 hours later personalized email → if they click but don't convert → SMS next day → if they purchase → stop the campaign

Customer.io is exceptionally well suited to this. Its Journeys product can trigger workflows from customer events and branch logic across email, SMS, push, and in-app messaging in real time. customer.io

You can send events such as:

• Viewed Pricing
• Signed Up
• Started Trial
• Invited Teammate
• Used Feature X
• Abandoned Checkout
• Purchased
• Subscription Cancelled

…and use those events to drive the messaging logic.

🥈 Braze if you're serious about scale

Braze is probably the more powerful choice if you're building a sophisticated customer-engagement operation. It supports real-time behavioral triggers, email + SMS/RCS, segmentation, cross-channel orchestration, experimentation, and AI-driven optimization. www.braze.com

It can also incorporate website behavior, including real-time triggers from actions such as purchases and abandoned carts. www.braze.com

The tradeoff is that Braze generally makes more sense when you have the budget, volume, and marketing complexity to justify an enterprise-grade platform.

🛒 If you're ecommerce: Klaviyo

If your use case is Shopify/ecommerce—browse → abandon cart → purchase → replenishment → winback—I'd put Klaviyo at the top.

What I'd choose

SaaS / web app: Customer.io
Ecommerce: Klaviyo
Large company / millions of users / complex omnichannel: Braze
Mid-market marketing team: Iterable

One important distinction: I'd specifically look for a platform with a first-class event model, rather than a traditional email marketing tool with some automation bolted on. Your architecture should be able to ingest arbitrary product events and use them immediately in journey logic.

If you tell me what kind of site you have (SaaS, ecommerce, marketplace, etc.), monthly active users, and roughly how many emails/SMS you'll send, I can narrow this to the best 2–3 options and compare pricing, event tracking, API quality, personalization, SMS, and ease of implementation.

If your core requirement is “user does X on my website → send a personalized email/SMS based on that behavior”, I’d shortlist these:

My recommendation: Customer.io

For a website/product where you have events such as:

• signed_up
• viewed_pricing
• started_checkout
• created_project
• invited_teammate
• used_feature_X
• subscription_expiring

Customer.io is probably the first one I'd evaluate. It is specifically designed around real-time behavioral data, segmentation, and multi-channel messaging, including SMS, push, and in-app. customer.io

You can build flows like:

User visits pricing page
        ↓
Wait 30 minutes
        ↓
Did they start checkout?
   ↙             ↘
 YES              NO
  ↓                ↓
Don't message     Email
                   ↓
              Wait 24 hours
                   ↓
              Still inactive?
                   ↓
                  SMS

And personalize the messages using the user's attributes and behavior.

When I'd choose the others

Braze — choose this if you're already at significant scale or need sophisticated orchestration across email, SMS, push, in-app, WhatsApp, etc. It can trigger journeys from real-time customer behavior and optimize channel/timing/content. www.braze.com

Klaviyo — my pick if you're an ecommerce business. Its strength is tying browsing, carts, purchases, products, and customer profiles together for email/SMS revenue automation. maestra.io

Iterable — worth considering if you're a larger marketing organization that wants visual journey building, experimentation, segmentation, and cross-channel campaigns. www.getvero.com

ActiveCampaign — attractive when you want marketing automation plus a conventional CRM rather than a deeply product-event-oriented lifecycle platform.

The important distinction

I'd avoid choosing based primarily on “email marketing” features. What you really want is a customer engagement/event automation platform.

The architecture should look roughly like:

Your website → event tracking → customer profile/event stream → journey engine → email/SMS provider

The critical capabilities are:

1. Real-time custom events from your site/backend
2. Identity resolution — anonymous visitor → logged-in user
3. Conditional journeys
4. Event-based triggers
5. Email + SMS in the same workflow
6. Per-user personalization
7. Frequency/campaign suppression
8. Conversion attribution
9. Webhooks/API
10. Good deliverability

For a SaaS/product-led website, my ranking would be:

1. Customer.io → 2. Braze → 3. Iterable → 4. ActiveCampaign

For ecommerce, I'd switch that to:

1. Klaviyo → 2. Braze → 3. Customer.io → 4. Iterable

If you tell me what your site does, roughly how many users you have, and whether you're SaaS/ecommerce/marketplace, I can narrow this to the best 2–3 options and compare pricing, event tracking, SMS costs, integrations, and implementation difficulty.

If your goal is “user does X on my website → automatically send a personalized email/SMS → branch based on what they do next,” I’d shortlist these:

1. Customer.io — my first choice

Customer.io

This is probably the closest match to what you're describing. You can define behavioral events such as:

• signed_up
• viewed_pricing
• started_trial
• created_project
• invited_teammate
• abandoned_checkout
• subscription_cancelled

…and use those events to start automated workflows. Customer.io explicitly supports automation triggers, multi-step workflows, and transactional email/SMS. docs.customer.io

For example:

User visits pricing → doesn't sign up → 2 hours later email → clicks email → SMS next day → signs up → stop campaign.

That's exactly the sort of workflow I'd use Customer.io for.

2. Braze — best if you're serious about lifecycle messaging

Braze

Braze is substantially more sophisticated. It supports real-time behavioral triggers, personalized email, SMS/RCS, web messaging, in-app messaging, experimentation, and AI-driven journey optimization. www.braze.com

I'd pick Braze if you're expecting millions of users, many channels, complex segmentation, and a dedicated lifecycle/CRM team.

For a startup with a relatively small user base, though, it can be more platform than you need.

3. Iterable — excellent enterprise alternative

Iterable

Iterable is particularly strong for visual, cross-channel journeys. It supports behavioral personalization, real-time segmentation, email, SMS, in-app, web, and other channels. iterable.com

I'd put it roughly in the same enterprise category as Braze.

What I'd choose

If you're a SaaS/startup: → Customer.io

If you're ecommerce: → Klaviyo

If you're an established company with a large lifecycle-marketing operation: → Braze

If you already use HubSpot heavily: → HubSpot

If you're enterprise and want sophisticated cross-channel orchestration: → Iterable or Braze

The important thing is that you don't just want an email marketing tool. You want a behavioral event → customer profile → journey orchestration system.

If you tell me your website stack (e.g. React/Next.js, WordPress, Shopify), approximate monthly active users, and whether you need transactional vs. marketing SMS, I can narrow this down to the best 2–3 options and compare pricing/integration complexity.

If your core requirement is “a user does X on my website → send them a personalized email/SMS → react to what they do next,” I’d shortlist these:

🥇 My pick: Customer.io

Customer.io is particularly well suited if you're building a SaaS, marketplace, subscription product, or other product where your own application events are the source of truth.

You can send events such as:

• signed_up
• completed_onboarding
• viewed_pricing
• started_checkout
• invited_teammate
• used_feature_X
• subscription_expiring
• inactive_for_7_days

…and use those events to branch a journey. Customer.io supports real-time event triggers, behavioral conditions, branching, personalization, email, SMS, push, and in-app messaging. customer.io

For example:

User visits pricing → doesn't purchase
→ wait 2 hours
→ personalized email
→ if they return to pricing → SMS
→ if they purchase → stop campaign
→ if they don't → send a different email 3 days later.

That's exactly the kind of workflow Customer.io is designed around. customer.io

🛒 If you're e-commerce: Klaviyo

Klaviyo would probably be my first choice instead.

Its flows can trigger from website/customer actions, and it has particularly strong email + SMS automation around browsing, carts, purchases, and post-purchase behavior. help.klaviyo.com

🏢 If you're big enough for enterprise tooling

I'd look at Braze or Iterable. They're excellent for sophisticated omnichannel customer journeys, particularly at large scale, but they're generally more infrastructure and budget than a startup needs. www.getvero.com

The important distinction

I'd choose based on where your behavioral data comes from:

• Your SaaS/app's events → Customer.io
• Shopify/e-commerce behavior → Klaviyo
• Millions of consumers + mobile app + many channels → Braze
• Large/mid-market omnichannel marketing team → Iterable
• CRM/sales automation is central → HubSpot

If you tell me what your website does, roughly how many users you have, and whether you need transactional SMS as well as marketing SMS, I can narrow this down to the best 2–3 options and compare their current pricing, APIs, event tracking, and implementation effort.

If your core requirement is “a user does X on my site → automatically send a personalized email/SMS sequence”, I’d shortlist these:

My picks

1. Customer.io — best overall for a SaaS/product website

This would be my first look if you're building a software product rather than an online store. You can send events from your website/backend and use them to enter people into highly specific automated campaigns. It supports both email and SMS in the same workflow, with dynamic personalization. docs.customer.io

For example:

User signs up → doesn't complete onboarding → email after 1 day → visits pricing page → SMS 2 hours later → upgrades → immediately stop the campaign.

Customer.io is particularly good when your own product events are the source of truth.

Customer.io

2. Klaviyo — best if you're e-commerce

Klaviyo makes this kind of automation exceptionally easy. Its flows can trigger on events such as product views, checkout starts, purchases, form submissions, and inactivity, with branching based on customer attributes. Email and SMS can live in the same flow. www.klaviyo.com

Klaviyo

3. Braze — best if you're operating at significant scale

Braze is the heavyweight option. Its Canvas system can react to real-time customer behavior and orchestrate email, SMS, push, in-app, WhatsApp, and other channels. It's especially compelling if you have a mobile app or sophisticated cross-channel engagement requirements. www.braze.com

Braze

What I'd choose

• SaaS / web app: Customer.io
• E-commerce: Klaviyo
• Consumer app with millions of users / complex lifecycle marketing: Braze
• You want CRM + sales + marketing in one system: HubSpot
• Small business wanting relatively easy automation: ActiveCampaign

One important distinction: don't choose based primarily on “email automation.” For your use case, the important capability is a robust event-based customer data model—you want to be able to send events such as signup, viewed_pricing, started_trial, invited_teammate, created_project, subscription_expiring, etc., then branch journeys based on those events.

If you tell me what your site/product does, roughly how many users you have, and whether this is B2B SaaS, B2C, or e-commerce, I can narrow this down to the best 2–3 and compare pricing, implementation effort, APIs, segmentation, and SMS costs.

If your core requirement is “someone does something on my site → send a personalized email/SMS based on that behavior”, I’d shortlist these:

My pick: Customer.io

For a typical software/SaaS company, I'd start with Customer.io. Its Journeys product is explicitly built around triggering workflows from customer events and can branch across email, SMS, push, and in-app messaging. It also supports personalization at send time based on behavior, preferences, and lifecycle stage. customer.io

For example:

User signs up → doesn't complete onboarding → wait 2 days → personalized email → if they return but don't activate → SMS → if they activate → stop the campaign.

You can feed it events such as:

• signed_up
• viewed_pricing
• started_trial
• invited_teammate
• created_project
• used_feature_x
• trial_ending
• subscription_cancelled

When I'd choose the others

Klaviyo is the better choice if you're selling physical products. Its flows can trigger from things like product views, checkout, purchases, forms, inactivity, etc., and combine email and SMS in the same customer journey. www.klaviyo.com

Braze is what I'd consider when you have a larger user base and need very sophisticated cross-channel orchestration. It supports real-time behavioral triggers, dynamic personalization, segmentation, email, SMS/RCS, push, in-app and web messaging. www.braze.com

HubSpot makes sense if you want marketing automation tightly integrated with CRM, sales pipelines, forms, lead scoring, etc., rather than primarily being a behavioral messaging engine.

The architecture I'd recommend

If you're building a SaaS/product, I'd generally do:

Your website/app → event tracking → Customer.io → email/SMS

rather than trying to make your web application directly send every message.

The important part is choosing a platform that understands events, not just email lists. You want to be able to say:

IF
  user viewed pricing
  AND hasn't started trial
  AND has email consent

THEN
  wait 1 day
  send personalized email

IF
  user returns to pricing
  AND still hasn't started trial

THEN
  send SMS

Customer.io is particularly well suited to this kind of event-driven workflow. customer.io

If you tell me what kind of site you have (SaaS, ecommerce, marketplace, lead-gen, etc.), roughly how many users you have, and whether you need transactional SMS as well as marketing SMS, I can narrow this down to the best 2–3 options and compare pricing, ease of implementation, APIs, and event tracking.

For personalized email + SMS triggered by user behavior on your site, the best software depends mostly on whether you’re a SaaS product, ecommerce store, marketplace, or content business. The strongest options are event-driven customer engagement platforms rather than traditional email marketing tools. www.sequenzy.comlearn.customer.io

Best overall for product behavior triggers (SaaS / apps)

Customer.io

Best for: “User signed up → didn’t complete onboarding → send email → if they still don’t activate → send SMS → if they convert → stop campaign.”

Strengths:

• Trigger from custom events (created_account, viewed_pricing, completed_step_2, etc.)
• Advanced segmentation and branching logic
• Email, SMS, push, and in-app messaging
• Good fit for engineering teams that can send product events www.sequenzy.comlearn.customer.io

Tradeoff:

• Requires more setup than drag-and-drop marketing tools.

Best for ecommerce (Shopify / DTC)

Klaviyo

Best for: abandoned carts, browse abandonment, post-purchase flows, VIP customers, replenishment reminders.

Strengths:

• Very strong ecommerce integrations
• Email + SMS in one platform
• Revenue attribution for campaigns
• Prebuilt behavioral flows www.clickpost.ai

Tradeoff:

• Less flexible than developer-first platforms for unusual product events.

Best enterprise / high-scale consumer apps

Braze

Best for: mobile apps, marketplaces, fintech, large consumer products.

Strengths:

• Real-time behavioral campaigns
• Email, SMS, push, and in-app orchestration
• Sophisticated personalization and lifecycle journeys www.clickpost.ai

Tradeoff:

• Expensive and usually requires dedicated marketing ops resources.

Best if you want CRM + automation together

HubSpot

Best for: B2B companies where website behavior should influence sales follow-up.

Examples:

• Visited pricing page 3 times → notify sales
• Downloaded a guide → enroll in nurture
• Became a qualified lead → start SMS sequence

Tradeoff:

• Can become expensive as automation requirements grow. zapier.com

Best simpler option for smaller teams

ActiveCampaign

Best for: businesses that need automation but don’t want to build a full event pipeline.

Good for:

• Email sequences
• Lead scoring
• CRM-triggered messaging
• Basic behavioral automation www.emailservicebusiness.com

My shortlist by scenario

A typical setup I’d recommend for a modern SaaS product:

Website/app events → customer data layer → Customer.io → email/SMS journeys

Track events like:

• Signup completed
• First key action completed
• Feature used
• Pricing page viewed
• Trial ending
• Subscription canceled
• User inactive for X days

Then create journeys:

• Activation onboarding
• Trial conversion
• Feature education
• Win-back
• Expansion/upsell

If you tell me what type of business you run (SaaS, ecommerce, marketplace, etc.), your monthly active users, and whether you already have a backend/event tracking system, I can narrow this to 2–3 best fits.

If your core requirement is “user does X on my site → send a personalized email/SMS → branch based on what they do next,” I’d shortlist these:

1. Customer.io — best overall for product-led / SaaS

Customer.io

This would be my first choice for most SaaS, marketplaces, and web apps. It is built around behavioral events: page views, cart abandonment, purchases, signups, custom events, etc. You can branch journeys, use event data for personalization, and send email + SMS + push + in-app from the same workflow. customer.iodocs.customer.io

Great for:
Viewed pricing → wait 2 hours → email → if signed up, stop → otherwise SMS → if clicked, sales notification

2. Klaviyo — best for ecommerce

Klaviyo

If you're selling physical products, I'd lean toward Klaviyo. Its flows natively handle things like viewed product, started checkout, purchased, form submission, and inactivity, with email and SMS in the same automation. www.klaviyo.com

Great for: browse abandonment, abandoned cart, post-purchase, winback, promotions, product recommendations.

3. Braze — best for large-scale / sophisticated engagement

Braze

Braze is the heavyweight option. It supports real-time behavioral triggers and orchestration across email, SMS/RCS, push, in-app, web messaging, and more, with sophisticated segmentation and experimentation. www.braze.com

Great for: millions of users, mobile apps, complex customer journeys, sophisticated personalization.

My quick recommendation

For your exact wording, I'd start with Customer.io. Its model is particularly well suited to sending messages based on arbitrary events from your website rather than just traditional marketing-list behavior. You can send your own events and event properties into the platform and use them to trigger and personalize journeys. docs.customer.io

If you tell me what kind of site you have (SaaS, ecommerce, marketplace, lead-gen, etc.), monthly active users, and roughly how many emails/SMS you'll send, I can narrow this down to the best 2–3 and compare pricing, event tracking, SMS costs, APIs, and ease of implementation.

If your core requirement is “a user does X on my website → send a personalized email/SMS → branch based on what they do next,” I’d shortlist these:

My pick: Customer.io

For a typical SaaS or web app, Customer.io is probably the best fit.

It is designed around event-driven journeys: you can trigger workflows from essentially any customer event, branch on behavior, and communicate through email, SMS, push, and in-app messaging. customer.io

For example:

User signs up → doesn't complete onboarding → wait 2 days → personalized email → if they visit pricing page → SMS → if they upgrade → exit campaign

That's the kind of workflow Customer.io is particularly good at.

If you're ecommerce: Klaviyo

I'd choose Klaviyo instead if you're selling physical products or running a consumer ecommerce business. Its flows can react to events such as product views, checkout, purchases, forms, price drops, and inactivity, with email/SMS/push in the same workflow. www.klaviyo.com

It's also considerably easier to get started with than something like Braze.

If you're big: Braze

Braze is the more enterprise-oriented choice. It's particularly strong when you need sophisticated cross-channel orchestration, real-time behavioral triggers, granular segmentation, experimentation, and very large volumes. Its SMS product supports real-time triggers based on customer behavior and backend events, alongside email, push, in-app, and other channels. www.braze.com

I'd avoid starting with Braze unless you actually need that level of sophistication.

The key question

The biggest differentiator isn't really email vs. SMS—all of these can do both. It's how you get your behavioral data into the platform.

For example, you want to be able to send events such as:

user_signed_up
project_created
invited_teammate
viewed_pricing
started_checkout
subscription_started
subscription_cancelled

and attach properties:

plan = "pro"
company_size = 25
project_name = "Acme"

Then your messaging platform turns those events into personalized journeys.

My ranking for most startups:

1. 🥇 Customer.io — best overall for behavioral/product-led messaging
2. 🥈 Klaviyo — best for ecommerce
3. 🥉 Braze — best for enterprise / high-scale consumer apps
4. Iterable — excellent for sophisticated lifecycle teams
5. HubSpot — best if you also want your CRM/sales/marketing system under one roof

If you tell me what kind of site you have (SaaS, ecommerce, marketplace, media, etc.), roughly how many users you have, and whether you need transactional as well as marketing messages, I can narrow this to the 2–3 best choices and compare pricing, event tracking, APIs, deliverability, and SMS costs.

For personalized email + SMS triggered by user behavior on your site, you’re looking for a customer engagement platform (not just an email newsletter tool). The best choice depends on your business model, traffic volume, and how sophisticated your triggers need to be. www.getvero.com

Best overall options

My picks by use case

If you run SaaS / a web app

1. Customer.io — best fit for event-driven messaging
2. Braze — if you have millions of users and enterprise needs
3. Intercom — if in-app chat/help is also central

Example triggers:

• User signs up → send onboarding sequence
• User visits pricing page 3 times → send sales-assist SMS
• User starts setup but stops → send reminder
• User reaches usage threshold → send upgrade nudge

If you run ecommerce

1. Klaviyo
2. Attentive
3. Omnisend

Example triggers:

• Viewed product → SMS reminder
• Added to cart → abandoned cart flow
• Purchased → review request + cross-sell
• No purchase in 90 days → win-back campaign

If you are technical and want maximum control

• Customer.io is usually the sweet spot: send custom events from your backend, define segments, and build journeys without building the messaging infrastructure yourself. getoden.com

Features I would require

Whatever you choose, make sure it supports:

• Custom event tracking (not just page visits)
• User profiles with attributes
• Branching journeys (“if/then” logic)
• Email + SMS in the same workflow
• Frequency limits (avoid annoying users)
• A/B testing
• Webhooks/API access
• Consent management for SMS compliance
• Integration with your analytics stack (Segment, Snowflake, Amplitude, etc.) www.getvero.comgetoden.com

A common architecture looks like:

Your website/app → event tracking → customer engagement platform → personalized email/SMS

If you tell me:

1. SaaS vs ecommerce vs marketplace,
2. monthly active users/customers,
3. your tech stack (Shopify, React, WordPress, custom app, etc.), and
4. approximate monthly budget,

I can narrow this to 2–3 best choices.

If your core requirement is “when a user does X on my website, send a personalized email/SMS based on that behavior”, I’d shortlist these:

\*SMS capabilities/costs depend on configuration.

🥇 My recommendation: Customer.io

For a website where you want triggers such as:

User signs up → waits 10 minutes → hasn't completed onboarding → email

User visits pricing page 3 times → send personalized email

User starts checkout → doesn't finish → SMS after 30 minutes

User uses Feature X → send an email showing them Feature Y

User becomes inactive → start a re-engagement sequence

Customer.io is probably the strongest fit. Its Journeys product is explicitly built around real-time events, attributes, segments, webhooks, branching logic, delays, and multi-channel messaging across email and SMS. customer.iowww.klaviyo.comwww.braze.com

You can essentially feed it an event stream from your site:

user_id: 123
event: viewed_pricing
plan: pro
company: Acme
pages_viewed: 7
last_login: today

…and use those properties to determine whether to message, what to say, which channel to use, and what happens next.

Customer.io

When I'd choose the others

Klaviyo — choose this if you're primarily e-commerce. Its flows can react to things like product views, checkout starts, purchases, forms, inactivity, and other customer behaviors, with email/SMS in the same flow. www.klaviyo.com

Klaviyo

Braze — choose this if you're operating at significant scale and want sophisticated cross-channel orchestration, experimentation, mobile/app messaging, and real-time personalization. It's powerful but generally much heavier than you need for an early-stage product. www.braze.com

Braze

HubSpot — attractive when the automation needs to interact heavily with your sales CRM, leads, deals, sales reps, and B2B lifecycle rather than just product behavior.

HubSpot

One important distinction

I would not start by looking for a traditional “email marketing” tool.

What you're describing is really a customer engagement / event-driven messaging platform.

The architecture you want is roughly:

Your website → behavioral events → customer engagement platform → decision/journey engine → email/SMS

The most important question is therefore whether the platform can ingest arbitrary events from your application, not just things like “opened email” or “filled out form.”

For that reason, Customer.io would be my first demo, especially for SaaS, marketplaces, subscriptions, or other product-led websites. Its current platform specifically supports event-triggered journeys and real-time behavioral conditions. www.braze.comcustomer.io

If you tell me what your website does (SaaS, e-commerce, marketplace, lead-gen, etc.), roughly how many users you have, and whether you're using Segment/PostHog/GA4/etc., I can narrow this to the best 2–3 options and compare pricing, implementation effort, and exact architecture.

If your core requirement is “when a user does X on my site, automatically send them a personalized email/SMS based on that behavior,” I’d shortlist these:

🥇 My pick: Customer.io

For a typical SaaS or web product, I'd start with Customer.io.

It is specifically designed around behavioral/event-driven messaging. You can send events such as:

• signed_up
• viewed_pricing
• started_checkout
• created_project
• invited_teammate
• used_feature
• subscription_expiring

…and then build workflows such as:

User visits pricing → wait 30 minutes → if they haven't signed up → personalized email → if they still haven't converted → SMS → stop when they purchase.

Customer.io supports email, SMS, push and in-app messaging, with branching logic based on events, segments and user attributes. customer.io

It's also relatively accessible compared with Braze/Iterable; current comparisons put Customer.io's entry pricing around $100/month, whereas Braze and Iterable are generally sales-led enterprise products. customer.iolearn.g2.commaestra.iowww.braze.comiterable.comwww.getvero.com

When I'd choose something else

Klaviyo → If you're running Shopify/ecommerce. Its strength is customer/product/purchase behavior and email + SMS revenue flows. maestra.io

Braze → If you have a large consumer product, lots of users, mobile apps, and need sophisticated real-time orchestration across email/SMS/push/in-app. It can trigger messages directly from real-time behavioral events. www.braze.com

Iterable → If you're a larger marketing organization that wants marketers to build sophisticated cross-channel journeys without engineering involvement. iterable.com

HubSpot → If you want the behavioral automation tightly connected to sales CRM, lead management, forms, pipeline, etc. I wouldn't choose it solely for event-triggered messaging.

The architecture I'd recommend

If you're building a SaaS/product, I'd generally do:

Website/app → event tracking → Customer.io → Email/SMS

For example:

User visits pricing
        ↓
Track "pricing_viewed"
        ↓
Customer.io
        ↓
Is user already a customer?
    ↙          ↘
   yes          no
   ↓             ↓
   stop      wait 2 hours
                  ↓
             Send personalized email
                  ↓
             Did they sign up?
              ↙       ↘
            yes        no
             ↓          ↓
            stop     SMS next day

The important thing is to choose a system where events are first-class inputs, rather than merely uploading lists and scheduling campaigns. Customer.io, Braze and Iterable are particularly strong in this model. learn.g2.comwww.getvero.com

If you tell me (1) SaaS/ecommerce/marketplace, (2) roughly how many users you have, and (3) whether you need SMS in the US only or internationally, I can narrow this to the best 2–3 options and compare actual pricing, implementation difficulty, event tracking, personalization, and SMS costs.

If your main requirement is “user does X on my website → automatically send a personalized email/SMS → branch based on what they do next,” I’d shortlist these:

1. Customer.io — best overall for SaaS / product-led sites

This would be my default recommendation if you're building a software product.

• Trigger journeys from arbitrary events: signup, feature usage, pricing-page visit, trial activity, purchase, etc.
• Strong branching and multi-step workflows.
• Email + SMS + push + in-app in the same journey.
• Particularly good when your engineering team can send rich event data into the platform. customer.io
• It has a relatively accessible entry point compared with enterprise platforms. learn.g2.com

Example:
Viewed pricing → wait 2 hours → if started trial, stop → otherwise email → if email clicked but no signup, SMS next day.

2. Klaviyo — best for ecommerce

I'd choose this over Customer.io if you're selling physical products online.

Its Flows can react to real-time behavior such as product views, checkout starts, purchases, forms and inactivity, with email and SMS coordinated in one workflow. www.klaviyo.com

It's especially strong for things like:

Viewed product → abandoned cart → purchase → post-purchase → replenishment → winback

3. Braze — best for large-scale consumer apps

Braze is the heavier-duty option when you have significant user volume and want sophisticated cross-channel orchestration.

It is designed around real-time customer behavior and can coordinate email, SMS, push, in-app and other channels. learn.g2.com

The tradeoff is cost and complexity—I'd generally avoid it for an early-stage company unless you have a strong reason to go enterprise.

4. Iterable — strong mid-market/enterprise alternative

Good if you want sophisticated cross-channel journey orchestration and experimentation without necessarily going all the way to Braze. It supports email, SMS, push and in-app journeys. www.getvero.com

My quick decision tree

For your wording specifically, I'd start with Customer.io. Its model is very close to what you're describing: send arbitrary user events from your site, use those events to enter journeys, personalize the message with event/user data, and branch the journey according to subsequent behavior. customer.io

If you tell me what kind of site you have (SaaS, ecommerce, marketplace, lead-gen, etc.), monthly active users, and roughly how many emails/SMS you expect to send, I can narrow this down to the best 2–3 and compare pricing, implementation effort, event tracking, personalization, and SMS costs.