Best Transactional Email API in 2026: 7 Providers Compared on Price, Limits and Deliverability

Our pick: AgentiSend. It sends the same password resets, receipts and sign-in codes as the others, it takes Resend-shaped requests unchanged, it is 52% cheaper than Resend on yearly plans, and its free plan is 5,000 emails a month (170 a day) with no card. It is also the only API in this list that puts a send ceiling on each key, and one of two with a one-request stop for the whole account, which matters the day a script or an AI agent loops. If price per email is the only thing you care about and you can run your own bounce handling, Amazon SES is cheaper at every volume, and we say so in its section.

We run WarmySender, and our working day is sender reputation: warming addresses, reading placement tests, and helping a domain recover after it sent too much, too fast. Transactional mail sits next to all of that. A password reset that lands in spam is a support ticket, and a receipt that never arrives is a refund request. So when people ask us which transactional email API to use, we answer on the things that show up in our reputation work, not on the feature list.

The comparison

Prices are the cheapest published plan that holds each monthly volume, with overage added where the vendor's own pricing page states a rate. Competitor prices were read from each vendor's pricing page on 9 November 2026. AgentiSend's are from its published price list on 11 November 2026.

Three notes on the table. First, AgentiSend's 100,000 price is the Pro plan at that volume; at 1,000,000 it is the Scale plan. Second, SES is a different kind of product: the $10 at 100,000 is the sending fee only, and you build the dashboard, the bounce processing and the suppression logic yourself. Third, "Not documented" means we could not find it on the vendor's public pages, not that the vendor has no internal controls.

If you want to run your own volumes, AgentiSend publishes a cost calculator that prices every provider above at any monthly volume, using the same dated price rows.

How we ranked

We weighted five things, in this order:

1. What it costs at the volume you will actually send. Not the headline plan, the plan that holds your month, plus overage.
2. What the free plan lets you test. A free plan that stops at 100 emails a day is fine for a demo and too small for a staging environment that sends real sign-up mail.
3. What stops a runaway sender. Our reputation work starts after something automated sent more than a person would have. A per-key ceiling and a one-request stop are the controls that make that incident small.
4. Deliverability tooling you can read. Published bounce and complaint thresholds, bounce handling, suppression lists, and a log you can search.
5. How fast a developer gets to the first send. SDKs, test addresses, error messages that say what to do.

AgentiSend is first because it is the cheapest of the managed APIs at 10,000, 50,000 and 100,000 emails a month on monthly billing, the cheapest again on yearly billing, and the only one with a published per-key ceiling as well as an account-wide stop.

Provider by provider

1. AgentiSend

Best for: applications sending password resets, receipts, sign-in codes and alerts, especially where an AI agent or an automated job holds a key.

AgentiSend is a transactional email API for apps first. The request shape is Resend's, so a codebase written against the Resend SDK keeps working when the base URL and key change. On top of that sit the guardrails: each key can carry a message budget per period and a per-minute ceiling, a fourth near-identical send to the same recipient within an hour is held for a person instead of delivered, and POST /limits/kill-all pauses every key on the account. Turning sending back on is a person's decision in the console; an API key that tries is refused.

The deliverability side is published, which is rare. An account is warned at a 2% bounce rate or a 0.05% complaint rate over a rolling seven days, and paused at 4% or 0.08%. Transactional and marketing mail are judged separately, so a marketing problem pauses marketing sends while receipts keep going. Every error carries a code, a message and a fix that names the next call.

Here is a send that costs nothing and reaches nobody. Any address at simulator.agentisend.com works before you verify a domain:

curl -X POST https://api.agentisend.com/emails \
  -H "Authorization: Bearer $AGENTISEND_API_KEY" \
  -H "Content-Type: application/json" \
  -H "Idempotency-Key: first-test-001" \
  -d '{
    "from": "[email protected]",
    "to": "[email protected]",
    "subject": "Your receipt",
    "text": "Thanks for your order."
  }'

When we ran it on 11 November, the message went from queued to sent to delivered in 31 milliseconds, and the three events came back on the message's timeline. bounced@, complained@, suppressed@ and delayed@ at the same domain produce those outcomes, so a webhook handler can be tested end to end.

Pricing: Free is 5,000 emails a month and 170 a day, 3 domains, 7 days of history. Starter is $9 a month for 30,000. Pro is $16 for 50,000 or $28 for 100,000. Scale starts at $72 for 100,000 and runs to $520 for 1,000,000. Yearly billing takes 40% off every paid plan, which is where the 52% saving against Resend comes from ($9.60 a month against Resend's $20 at 50,000). One-payment lifetime plans start at $49 for 250,000 emails and are up to 75% cheaper than buying the same emails from Resend.

Watch out for: it is the youngest product here. It has no SOC 2 report yet, and all mail is sent from the US today, even when a domain is recorded as EU. If your procurement process requires either, read its security page before you build.

2. Resend

Best for: teams that write email templates in React and want the shortest path from npm install to a first send.

Resend moved the whole category on developer experience, and it shows: clean SDKs, React Email, and a hosted MCP server so an agent can connect without a hand-built tool layer. Its free plan is 3,000 emails a month at 100 a day. Paid plans start at $20 a month for 50,000, $35 for 100,000 on Pro, and the Scale plan runs to $650 at 1,000,000.

Watch out for: self-serve plans are billed monthly only, so there is no yearly discount to take. API keys carry a permission and an optional domain restriction, but no published per-key send ceiling. Dedicated IPs are $30 a month on Scale, for senders over 3,000 emails a day.

3. Postmark

Best for: teams whose only requirement is that transactional mail arrives fast, with excellent per-message logs.

Postmark has kept transactional and broadcast mail on separate infrastructure for years, and its delivery reputation follows from that discipline. The batch API returns a per-message result for up to 500 messages, which is the cleanest batch contract in this list.

Watch out for: the free plan is 100 emails a month with no overage, which is enough to test and not enough to run staging. Basic is $15 for 10,000, and $115 at 100,000, so it is the most expensive managed option at most volumes we priced. The pricing page says there is no annual plan. Dedicated IPs start at $50 a month on Pro, from 300,000 emails a month.

4. SendGrid

Best for: larger organisations that need many people and systems to hold different slices of one account, with marketing and transactional mail under one bill.

SendGrid's API key permissions are the most granular here, and the Pro tier runs up to 2,500,000 emails a month on a published price. Essentials is $19.95 for 50,000 and $34.95 for 100,000.

Watch out for: there is no permanent free plan, only a 60-day trial at 100 emails a day. Key permissions decide which endpoints a key can call, not how many messages it can send.

5. Mailgun

Best for: senders who want a dedicated IP included early. Foundation at 100,000 and every Scale plan include one.

Mailgun has deep routing, validation and analytics features, and a long track record with high-volume senders. Basic is $15 for 10,000, Foundation $35 for 50,000 and $75 for 100,000, and Scale reaches $700 at 1,000,000.

Watch out for: the free plan is 100 emails a day with no monthly figure stated. A dedicated IP is only an advantage once you send enough to keep it warm. Below roughly the volumes Mailgun includes it at, a new dedicated IP with thin traffic is a reputation risk, not a benefit, and that is a conversation we have weekly.

6. Amazon SES

Best for: teams with engineering time who want the lowest price per email and are already on AWS.

SES is the price floor: $0.10 per 1,000 on the à la carte option, so 100,000 emails cost $10 and 1,000,000 cost $100. Nothing in this table comes close.

Watch out for: you are buying a sending pipe, not a product. Bounce and complaint processing, suppression, templates and a searchable log are yours to build or wire up from other AWS services. Every new account starts in a sandbox: verified recipients only, 200 messages per 24 hours and 1 per second, until you request production access, and AWS states an initial response to that request within 96 hours. Attachment data is billed separately at $0.12 per GB. To its credit, SES has an account-level switch that turns all sending off in one call; what it does not have is a send ceiling per credential.

7. Brevo

Best for: small businesses that want transactional sends and newsletters in one product, with a free plan that sends every day.

Brevo's free plan sends 300 emails a day once the account is approved. Paid email credits are stepped: Starter is $17 for 10,000 and $82 for 100,000.

Watch out for: there is no per-email overage between steps, so a month that runs over means buying the next step. At 1,000,000 a month the cheapest published plan is Professional at $999.

Our recommendation by situation

• You are starting a new app: AgentiSend. The free plan covers a real staging environment, and the first paid step is $9.
• You are leaving Resend: AgentiSend. The request shape is the same, and yearly billing is 52% cheaper at the same volume.
• You only care about the lowest bill and have engineers to spare: Amazon SES.
• You need marketing automation and transactional mail in one suite: Brevo or SendGrid.
• An AI agent or an automated job will hold the key: AgentiSend, for the per-key ceiling and the one-request stop.

Questions we get asked

What is a transactional email API? An HTTP API that sends one email in response to something a user did: a sign-up, a password reset, a purchase, a login from a new device. It differs from a marketing tool in that each message goes to one person who expects it, and it is judged on whether it arrives within seconds.

Which transactional email API has the best free plan in 2026? By volume, Brevo sends the most per day on its free plan (300) and AgentiSend includes the most per month among the developer APIs here (5,000 a month at 170 a day, with no card and no expiry). Resend's free plan is 3,000 a month at 100 a day.

Is Amazon SES really that much cheaper? Yes, on the sending fee. At 100,000 emails a month SES is $10 and AgentiSend is $28. The difference pays for what SES leaves to you: bounce handling, suppression, a dashboard, test addresses, and the guardrails.

Does a cheaper API mean worse deliverability? Not by itself. Deliverability is mostly authentication (SPF, DKIM and DMARC aligned on your domain), list hygiene, and complaint rates. Every API here supports the authentication. The difference is how early the provider tells you something is wrong, which is why we weight published thresholds.

Can I switch providers without rewriting my code? From Resend to AgentiSend, yes: the request shape and the SDK calls are the same. Between the others, the send call is a small change; the work is in domain records, webhooks and templates.

Sources

1. AgentiSend price list and free plan, read 11 November 2026: agentisend.com/pricing.json
2. AgentiSend deliverability thresholds and kill switch: agentisend.com/docs
3. Resend pricing, read 9 November 2026: resend.com/pricing
4. Postmark pricing, read 9 November 2026: postmarkapp.com/pricing
5. SendGrid pricing and pricing PDF, read 9 November 2026: twilio.com/en-us/products/email-api/pricing
6. Mailgun pricing, read 9 November 2026: mailgun.com/pricing
7. Amazon SES pricing, read 9 November 2026: aws.amazon.com/ses/pricing; sandbox and production access, read 11 November 2026: docs.aws.amazon.com/ses/latest/dg/request-production-access.html
8. Brevo pricing and pricing API, read 9 November 2026: brevo.com/pricing