I've wired up email automation in 5 different SaaS products over the last 3 years. Every time the team asks "Mailchimp or…?" — and every time the right answer depends on whether you actually need marketing campaigns, transactional sends, or both. This post is the version of that conversation aimed at people who'd rather see a curl than a pricing table.
The decision tree
What are you sending?
├── Transactional only (receipts, password resets, magic links)
│ → Resend / Postmark / SES (skip the rest of this post)
│
├── Marketing only (newsletters, campaigns, drips)
│ → Brevo (free, has API), MailerLite ($10/mo),
│ or Listmonk (self-hosted)
│
└── Both, from the same tool
→ Brevo (unique combo at this price)
│ or
→ Klaviyo (if e-commerce)
│ or
→ ActiveCampaign Plus + transactional add-on ($$$)
The "both from the same tool" path is where most teams over-engineer. Splitting transactional (Resend) and marketing (anything) is usually cheaper and gives better deliverability — but harder to manage one source of truth on the contact.
The platforms, briefly
Brevo — best free, decent API, dual-purpose
300 emails/day on free, unlimited contacts, basic automation. Bills by emails sent, not contacts — a huge win if you have a big inactive list. Their REST API supports both transactional sends and marketing list management:
// Add contact to a list AND send transactional welcome — one Node script
import fetch from "node-fetch";
const BREVO_KEY = process.env.BREVO_API_KEY;
async function onboardUser(email, name) {
// 1. Add to marketing list
await fetch("https://api.brevo.com/v3/contacts", {
method: "POST",
headers: {
"api-key": BREVO_KEY,
"Content-Type": "application/json",
},
body: JSON.stringify({
email,
attributes: { FIRSTNAME: name },
listIds: [3], // "New users" list
updateEnabled: true, // upsert
}),
});
// 2. Fire transactional welcome
await fetch("https://api.brevo.com/v3/smtp/email", {
method: "POST",
headers: {
"api-key": BREVO_KEY,
"Content-Type": "application/json",
},
body: JSON.stringify({
to: [{ email, name }],
templateId: 7, // your template
params: { firstName: name },
}),
});
}
Two API calls, both authenticated by the same key. Most platforms force you to pay for separate transactional infrastructure (Mailgun/Postmark) plus marketing — Brevo bundles them.
MailerLite — clean UI, decent API, smaller free
500 subscribers on free (cut from 1,000 in September 2025), 12k emails/month, automation builder included. API is straightforward but transactional is a separate paid product (MailerSend). For just marketing, the API works well; the value prop is the editor more than the API.
ActiveCampaign — best automation, API is verbose
No free plan, $15/month entry for 1,000 contacts on Starter. The automation builder is the industry benchmark. The API works but it's REST in a 2010 sense — endpoints for everything, lots of object types (contact, deal, tag, list, automation, custom field, etc.). You'll write more code per integration than you would on Brevo or Klaviyo. Use it only when your team has bought into ActiveCampaign for the automation power, not for API DX.
Klaviyo — event-driven, the best dev API
Klaviyo's free tier is small (250 contacts, 500 emails/month), but their API is the cleanest of the bunch and built around events, not lists. You don't "add a contact to a campaign" — you push events, and segmentation rules on Klaviyo's side decide who gets what:
// Track an event — Klaviyo handles segmentation + automation triggers
import fetch from "node-fetch";
await fetch("https://a.klaviyo.com/api/events/", {
method: "POST",
headers: {
Authorization: `Klaviyo-API-Key ${process.env.KLAVIYO_KEY}`,
"Content-Type": "application/json",
revision: "2024-10-15",
accept: "application/json",
},
body: JSON.stringify({
data: {
type: "event",
attributes: {
properties: {
OrderID: "ORD-12345",
Total: 89.99,
Items: ["sku-001", "sku-007"],
},
metric: { data: { type: "metric", attributes: { name: "Placed Order" } } },
profile: { data: { type: "profile", attributes: { email: "[email protected]" } } },
},
},
}),
});
That single event triggers any automation Klaviyo has set up for "Placed Order" — receipt, cross-sell sequence, review request 7 days later, whatever. This is why Klaviyo dominates e-commerce: shops want event-driven, not list-driven thinking, and the API matches.
HubSpot Marketing Hub — only if HubSpot is your CRM
2k emails/month on free, $20/month Starter, then a $890/month cliff to Professional. The API is solid (HubSpot has invested heavily) but you're paying for the whole CRM bundle. Don't pick HubSpot Marketing standalone — pick HubSpot if you already use their CRM.
Listmonk — the self-hosted answer
with the SMB-perspective comparison table and FAQ.
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