The best email API for developers depends on the job

Five different jobs#

Search for the best email API and you will find the same ten names ranked against each other. The ranking hides the real question: which job are you hiring it for? Five jobs get called “email API”, and the best product for each is different.

• Sending password resets, receipts and notifications from your app.
• Receiving mail at your domain: replies, support@, forwarding into your app.
• Reading your users' own Gmail or Outlook mailboxes, with their consent.
• Testing that your sign up and reset emails arrive and contain the right link.
• Agents: inboxes an AI agent owns, to sign up, read a code and reply.

Runearc is currently free. The paid Runearc prices below are planned and come from our pricing page; billing is not live yet.

1. Sending app email#

This is the classic transactional job, and the field is crowded. Resend is known for a clean API and React Email templates. Postmark is known for focusing on transactional mail and deliverability. SendGrid and Mailgun are long established and cover high volume plus extras such as templates, routing and validation. Amazon SES is the cheapest at scale, but you assemble the dashboard, bounce handling and suppression lists yourself.

Whichever you choose, the work that decides whether mail lands is the same: authenticate your own domain. Our SPF, DKIM and DMARC guide shows the records. Runearc can send app email from a verified domain too, over REST or SMTP, and the Resend alternative page shows a migration that keeps your request body.

2. Receiving mail#

Receiving is where products differ most. Some treat inbound as a webhook that posts a parsed message to your URL; others store mail in an inbox you can list and search. Before you commit, check three things: whether received mail counts against your sending quota, how long it is kept, and whether you can read it later or only get one webhook call.

Runearc stores inbound mail in inboxes you read over REST, IMAP or MCP, and received mail does not eat into the sending allowance. That is the point of the product, so it is also its strongest case.

3. Reading user mailboxes#

If your product needs to read or send from your customers' own Gmail or Outlook accounts, you need an API built on top of those providers, such as Nylas. A sending API cannot do this, and neither can Runearc: it gives you new inboxes, not access to existing ones.

4. Testing email flows#

End to end tests need a fresh address per run and a way to wait for the message. Mailosaur is a complete testing product. A catch-all inbox on a domain you control does the same core job: each test invents an address and blocks until the mail arrives. See catch-all inboxes for end to end tests and the Mailosaur alternative.

5. Inboxes for AI agents#

An agent needs its own address, a blocking wait instead of polling, codes and links already extracted, and a tool interface such as MCP. AgentMail and Runearc are built for this. The next post goes through what to check: the best email API for AI agents.

A checklist before you commit#

• Can you use your own domain on the plan you will actually pay for?
• How is inbound priced, and how long is mail retained?
• Do you get logs, bounces and webhooks you can verify?
• Is there an SMTP option, for tools that cannot call an API (Supabase Auth, for one)?
• What happens to your data and your domain if you leave?
• Is the provider old enough to trust for the thing you are putting through it?

On that last point we say it plainly: Runearc is new, and access is limited while we finish the product. For a payment receipt that must arrive, use the provider you trust. For agent inboxes, shared support mail and test inboxes, try Runearc free.

If you are leaving SendGrid specifically, the SendGrid alternative page lists what changes.