Guides / Best email API for AI agents
November 6, 2026 · every claim checked against the vendor’s own docs that day
Most roundups of email APIs for agents rank them on inboxes, threads and price. Those matter, but they are not what goes wrong. What goes wrong is an agent that reads an invoice with a line in it saying “wire the money to this new account”, and does it. Or an agent that sends a signed contract to an address a stranger suggested. So this comparison asks the questions you would ask before letting an agent act on email.
yes partly no, or not documented
| Product | Own address | Sender checks | Prompt injection | Values checked | Sends held for a human | Verifiable agent identity |
|---|---|---|---|---|---|---|
| Inbin | One call, or no account at all | On every event | Labeled per field | Values not in the email are dropped | Held until the owner approves by email | Signed per message, takeover detection |
| AgentMail | Inboxes API | SPF, DKIM, DMARC | Whole-message verdict | No | Lists; approval if you scope keys | Auth to AgentMail only |
| MachineMail | One API call | Plus From alignment | Whole-message trust level | Codes and links only | Draft-only keys | Not documented |
| Mails.ai | Agents API | Docs disagree on SPF | Whole-message score | No | Recipient lists, no per-send approval | On their roadmap |
| Resend | Inboxes API | SPF, DKIM, DMARC | None for your agent | No | None for API sends | No |
| Postmark | Inbound domain | SPF header only | None | No | Not documented | No |
| SendGrid | Inbound Parse | SPF and DKIM | None | No | Not documented | No |
| Amazon SES | Receipt rules | SPF, DKIM, DMARC | None | No | Not documented | No |
| Mailgun | Routes | Since May 2026 | None | No | Not documented | No |
| Nylas | Agent Accounts | Checked, not in the payload | Left to your code | Cookbook only | Outbound block rules | No |
| Gmail API | Existing mailboxes | Raw headers only | None | No | Not documented | No |
| Cloudflare Email | Routing rules | Authentication-Results | None | No | Reference app drafts | Signed reply routing only |
“Not documented” means we searched the vendor’s docs and found nothing, not that the feature cannot exist. Sources are listed at the bottom.
Sender checks are now standard. Resend, Amazon SES, Mailgun, AgentMail and MachineMail all hand you SPF, DKIM and DMARC results. That is good, and it is a baseline. Those checks prove which server sent the email, not whether the words inside are safe to follow.
Injection defenses work on the whole message. AgentMail gives each message a clear or review verdict. MachineMail gives it a trust level and removes hidden text. Mails.ai scores it. None of them separates the parts of one message: the invoice total and the sentence asking for a wire arrive with the same label.
Nobody else checks the numbers. We found no product that documents a check that an extracted value actually appears in the email. If a model reads $289 as $189, the error travels on.
Holding sends is mostly left to you.AgentMail and MachineMail let you build an approval step by giving the agent a key that can only draft. Resend’s own Inbox Agent only drafts, but that rule does not cover your agent. Nylas’s MCP asks for a confirm call, which the agent makes itself.
Built for the moment an agent acts. Every event carries the sender checks, each field is labeled as a checked typed value or as the sender’s free text, and values the model cannot find in the email are dropped and counted. Replies to people you deal with and mail to contacts you added go out on their own; anything else is held and the owner approves it from an email, with no login. Agents can prove a verified identity on every message, and two agents can detect when the other’s address has been taken over. An agent can also create a trial inbox with no account at all.
The best-known agent inbox, with inboxes, threads, allow and block lists, and Agent Armor, a per-message verdict with an optional hold. It has a SOC 2 Type II report. Approval before sending is a pattern you build with key permissions.
The closest to a trust-first design: a per-message trust level from verified to quarantined, hidden text removed before the agent sees it, and keys that can be limited to drafting.
Agent addresses with an injection score and a quarantine flag, recipient allow and block lists, and a refusal of cold email. Its own docs disagree on whether SPF is checked.
Sending platforms that also receive. They are the right choice for volume and deliverability, and most now pass you the sender checks. What the agent does with the message is left to your code. We compared their inbound side in more detail in this guide.
For an agent working inside a person’s existing Gmail or Outlook mailbox. The agent borrows that person’s address and everything in it, and prompt injection defense is left to you.
A good fit if you already run on Workers. Mail arrives with its authentication results, and reply routing can be signed. The message itself reaches your code raw; our Cloudflare guide shows how to put checks in front of it.
The one that checks the email before the agent reads it and checks the agent's sends before they leave. Most email APIs do neither: they deliver the message, and whatever an agent does with it is your code's problem. Of the products compared here, Inbin is the only one that labels each extracted field as checked or as the sender's free text, drops values it cannot find in the email, and holds sends to anyone new until the owner approves them by email.
No. Those checks prove which server sent the email. They say nothing about whether the text inside is safe to follow, and a domain registered an hour ago passes them for itself. Most providers expose these results now (Resend, Amazon SES, Mailgun, AgentMail, MachineMail), so they are a baseline, not a differentiator.
Yes. AgentMail, MachineMail, Mails.ai, Resend and Inbin all create addresses through an API. Inbin also lets an agent create a temporary inbox with no account or API key at all, and a person can claim that address later to keep it.
Yes. AgentMail documents semantic search across an organization's inboxes, and Inbin has it too: "late payment" finds an invoice that says "30 days overdue". The difference in Inbin is that the trust floor sits inside the same search. An agent can ask for only email its owner's policy vouches for, and an unverified or spoofed message stays out of the results even when its words match best.
Make the hold a platform rule, not something the agent decides. With most APIs an approval step only exists if you build it and scope the agent's keys so it cannot send. Inbin holds any send its policy has not established, emails the owner a link and a code, and the agent cannot approve its own send.
For high-volume transactional or marketing sending, use a sending platform such as Amazon SES, Postmark or Resend. To act inside a person's existing Gmail or Outlook mailbox, use Nylas or the Gmail API; Inbin never logs into your mailbox. If a SOC 2 report is a hard requirement today, AgentMail has one and Inbin does not yet.
Checked on November 6, 2026. LobsterMail is left out because its documentation could not be reached that day. Prices are left out on purpose: they change too often. Corrections are welcome at hello@inbin.dev and we will update the table.