Best email APIs
Best Email API and Transactional Email Service
The best email API and transactional email services compared on price, domains, inbound email, SDKs and AI-agent support, with figures checked in October 2026.
The best email API depends on the job. SendHQ is best for AI agents and products that send and receive, at $9 a month for 50,000 emails and 25 domains. Resend is best for developer experience, Postmark for transactional-first teams, Mailgun for many domains and inbound routing, and SendGrid for subusers and SSO. This list compares the five on price, free tier, domains and inbound email, using figures read on each vendor's own pages in October 2026.
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Key takeaways
- SendHQ is best for AI agents and products that need to send and receive: an inbox and sending identity per product or agent, an MCP server, a CLI, and $9 a month for 50,000 emails and 25 domains.
- Resend is best for developer experience: SDKs in nine documented languages, React Email, and a free-forever plan with 3,000 emails a month.
- Postmark is best for a transactional-first service with long retention: up to 365 days on Pro and Platform, and inbound processing from Pro.
- Mailgun is best for many domains and inbound routing on a mid plan: 1,000 custom domains from $35 a month.
- SendGrid is best when you need subusers, SSO and a dedicated IP bundled into one plan.
What an email API is (and is not)
A transactional email API sends messages triggered by a person's action: password resets, receipts, verification links, alerts. You call an HTTPS endpoint, or an SMTP relay, from your server, and the provider delivers the message from a domain you have verified. A good one also returns delivery, bounce and complaint events, and handles suppression. It is not a marketing tool for campaigns to purchased lists, and it is not a mailbox you read by hand. Our guide to transactional email APIs covers the basics, and transactional vs marketing email explains the difference.
How we compared
We ranked on criteria you can check on each vendor's own pages. As of October 2026. Prices are in US dollars and change; confirm on the vendor page before you buy.
- Price at entry volume and what the free tier allows
- Custom sending domains included
- Inbound email support
- Developer tooling: SDKs, SMTP relay, MCP server
- Fit for AI agents that need their own inboxes
The best email APIs, ranked
1. SendHQ, best for AI agents and inbound inboxes
SendHQ is an email API where each product, service and AI agent gets its own inbox and sending identity in one workspace. Mail sent and mail received share one place, with threads, labels and delivery events. You can reach it through the REST API (OpenAPI 3.1), a hosted MCP server for Claude, ChatGPT, Grok and Muse, or the sendhq command-line tool. Domain setup is one-click for Cloudflare and Domain Connect providers.
Pricing is published and flat: Starter is $9 a month for 50,000 emails and 25 verified domains, Scale is $39 a month for 200,000 emails and 100 domains, and annual billing takes 10% off. Every plan accepts USDC, and Scale Annual can be paid in Monero. There is no free plan; new workspaces get an integration trial. SendHQ has no SMTP relay, so it is an HTTPS API, MCP and CLI product only. Read more about inboxes for AI agents and the MCP server.
Best suited for: AI agents, and products with several domains that need to send and receive from one API.
2. Resend, best for developer experience
Resend's Free plan is $0 with 3,000 emails a month, a 100-a-day cap and 3 domains. Pro is $20 a month for 50,000 emails with 10 domains, and Scale is $90 a month for 100,000 emails with 1,000 domains. Extra emails are $0.90 per 1,000 with pay-as-you-go. It documents SDKs for Node.js, PHP, Ruby, Python, Go, Rust, Elixir, Java and .NET, supports SMTP, React Email, receiving email and scheduled sends, and runs a hosted MCP server. Plans are monthly only.
Best suited for: teams that want polished SDKs and a free plan that does not expire. See the Resend alternative guide and Resend vs SendGrid.
3. Postmark, best for transactional-first teams with long retention
At 10,000 emails a month, Postmark lists Basic at $15 (5 custom domains, 45-day data retention), Pro at $16.50 (10 domains, retention customizable up to 365 days, inbound email processing) and Platform at $18 (unlimited domains). Extra emails are $1.80, $1.30 and $1.20 per 1,000 respectively. A free Developer plan includes 100 emails a month and does not expire. Postmark's price moves with volume, so use its slider.
Best suited for: teams that want a transactional-focused service and need inbound email and long retention on a modest plan. Compare it in our Postmark alternative guide.
4. Mailgun, best for many domains and inbound routing
Mailgun's Free plan is $0 with 100 emails a day. Basic is from $15 a month for 10,000 emails, Foundation is $35 a month for 50,000 emails (first month free) with 1,000 custom domains and full inbound routing, and Scale is $90 a month for 100,000 emails with SAML SSO and dedicated IP pools. It offers both a REST API and an SMTP relay, hosts in the US and the EU from one account, and publishes an MCP server on GitHub.
Best suited for: teams that need many sending domains and webhook-style inbound routing. See Mailgun alternatives and SendGrid vs Mailgun.
5. SendGrid, best for subusers, SSO and bundled dedicated IPs
SendGrid has a 60-day free trial at 100 emails a day. Essentials starts at $19.95 a month and Pro starts at $89.95 a month; Premier is custom and costs more than Pro. Pro lists a dedicated IP, subuser management, single sign-on, 1,000 additional teammates and 2,500 email validations. It supports SMTP and API sending and receives mail through the Inbound Parse webhook.
Best suited for: larger teams that manage many sub-accounts and want a dedicated IP in the plan. See SendGrid alternatives and our migration notes.
Comparison table
| Service | SendHQ | Resend | Postmark | Mailgun | SendGrid |
|---|---|---|---|---|---|
| Free option | Integration trial, no free plan | Free plan, 3,000 emails a month | Free plan, 100 emails a month | Free plan, 100 emails a day | 60-day trial, 100 emails a day |
| Entry paid plan | $9 a month, 50,000 emails | $20 a month, 50,000 emails | $15 a month, 10,000 emails | $15 a month, 10,000 emails | From $19.95 a month |
| Custom domains on entry plan | 25 | 10 (Pro) | 5 (Basic) | 1 (Basic) | Not stated on pricing page |
| Inbound email | Yes, inboxes with threads and labels | Yes | Pro and Platform | Inbound routes: 5 on Basic, full on Foundation | Inbound Parse webhook |
| SMTP relay | No | Yes | Yes | Yes | Yes |
Which one fits your use case
| If you need | Pick | Why |
|---|---|---|
| Inboxes for AI agents and a vendor MCP server | SendHQ | Inbox and sending identity per agent, hosted MCP, CLI |
| The lowest price for 50,000 emails with many domains | SendHQ | $9 a month with 25 domains, against $20 and 10 domains on Resend Pro |
| SDKs and a free plan that does not expire | Resend | Nine documented SDK languages, free-forever plan |
| Inbound email and long retention on a small plan | Postmark | Pro adds inbound and up to 365 days retention |
| An SMTP relay as well as an API | Mailgun, SendGrid, Resend or Postmark | SendHQ does not offer SMTP |
| Subusers, SSO and a dedicated IP in one plan | SendGrid | Pro bundles them |
Try SendHQ
If you are choosing an email API for an AI agent, or for several products that need to send and receive, start with the SendHQ plans. Starter is $9 a month, and the integration trial lets you test the API before you pay.
Sources and dates
Competitor prices and limits were read on these pages on . SendHQ details come from its own live site.
Questions teams ask
What is the best email API for developers?
It depends on the job. Resend is a strong choice for SDKs and React Email, and SendHQ is built for AI agents and for sending and receiving across several domains. Compare price and domain limits at your volume.
What is the best transactional email service for a small startup?
For a small startup, compare the entry plans: SendHQ Starter at $9 a month for 50,000 emails, Resend Pro at $20 for 50,000, and Postmark or Mailgun at $15 for 10,000. Pick based on your volume and whether you need inbound email.
Which email API has a free plan?
Resend has a free-forever plan with 3,000 emails a month, Mailgun has a Free plan with 100 emails a day, and Postmark has a free plan with 100 emails a month. SendGrid has a 60-day trial. SendHQ has no free plan, but new workspaces get an integration trial.
Which email API is best for AI agents?
SendHQ gives each agent its own inbox and sending identity, with a hosted MCP server and a CLI. Resend and Mailgun also publish MCP servers.
Do I need an SMTP relay?
Only if you have software that can send over SMTP but not over HTTPS. Resend, Postmark, Mailgun and SendGrid all offer one. SendHQ does not.
Can I send marketing email through a transactional API?
Only if the provider and your recipients' consent allow it. Keep promotional mail separate from receipts and password resets. SendHQ sells separate Transactional and Marketing plan variants.