Email for AI agents

What an agent needs from email

• To hear about new mail at once, not by polling a mailbox every minute.
• To read the whole conversation: earlier messages, attachments, what was already answered.
• To send from a real address on your domain, signed with DKIM so replies don't land in spam.
• A person who can look over its shoulder, correct an answer or take over a conversation.
• Limits, so a loop in your code can't send ten thousand emails.

A sending API covers the third point only. A normal mailbox covers the first four, but its API is built for one person's account, not for software. Mektup is a mailbox built to be used by both.

How it works

1. Someone writes to support@yourcompany.com. Mektup's mail servers receive it, check it for spam and store it in the mailbox.
2. Mektup posts the message to your agent's webhook as soon as it is stored, signed with your webhook secret.
3. Your agent reads the message, and if it needs more, the thread and attachments through the API.
4. It replies with one API call or the send_email MCP tool, passing the message's id as inReplyTo. The reply goes out from support@, DKIM-signed, in the same thread, and is saved in Sent.
5. Your team sees the whole conversation in webmail or their own mail app.

Set it up

1. Add your domain in Mektup (dashboard, API or the create_domain tool) and add the MX, SPF, DKIM and DMARC records it returns at your DNS host.
2. Create the mailbox: POST /v1/domains/yourcompany.com/mailboxes with {"localPart": "support"}.
3. Point its webhook at your agent: PUT /v1/domains/yourcompany.com/mailboxes/support/webhook. The response shows the signing secret once.
4. Handle the webhook and reply. A complete Node.js example:

import express from "express";
import crypto from "node:crypto";

const app = express();

app.post("/inbound", express.raw({ type: "application/json" }), async (req, res) => {
  // 1. Check that the call came from Mektup
  const expected = crypto.createHmac("sha256", process.env.MEKTUP_WEBHOOK_SECRET)
    .update(req.body).digest("hex");
  const got = String(req.get("X-Mektup-Signature") || "").replace("sha256=", "");
  if (got.length !== expected.length ||
      !crypto.timingSafeEqual(Buffer.from(got), Buffer.from(expected))) {
    return res.sendStatus(401);
  }
  res.sendStatus(200); // answer within 8 seconds, do the work after

  // 2. Let your agent decide what to say
  const { mailbox, message } = JSON.parse(req.body);
  const answer = await runAgent(message); // your model call

  // 3. Reply from the same address
  await fetch("https://api.usemektup.com/v1/emails", {
    method: "POST",
    headers: {
      Authorization: `Bearer ${process.env.MEKTUP_API_KEY}`,
      "Content-Type": "application/json",
    },
    body: JSON.stringify({
      from: mailbox,
      to: message.from,
      subject: `Re: ${message.subject}`,
      text: answer,
      inReplyTo: message.id, // keeps the reply in the same thread
    }),
  });
});

app.listen(3000);

An open-source version with a model call and a human safety net is on GitHub: mektup-inbox-agent.

Or let the agent do it through MCP

Agents in Claude Code, Claude Desktop, Cursor, Lovable or Replit can use Mektup as tools: create_domain, get_domain_records, create_mailbox, set_mailbox_webhook, list_messages, get_thread, send_email and the rest of the API. Add the remote server to your client:

{
  "mcpServers": {
    "mektup": {
      "url": "https://mcp.usemektup.com/mcp",
      "headers": { "Authorization": "Bearer mek_live_..." }
    }
  }
}

Clients that support OAuth only need the URL https://mcp.usemektup.com/mcp and sign in with your Mektup account. More on the MCP server.

Limits that protect you

Limits hold on every way out: the API, MCP, SMTP, forms, forwards and auto-replies. Over a limit the API answers 429 and nothing is sent.

Other ways to give an agent email

AgentMail sells inboxes for agents priced per inbox; Hostinger Agentic Mail adds an API and MCP to Hostinger mailboxes; Nylas Agent Accounts host a mailbox inside the Nylas API; Resend and Postmark parse inbound mail into webhooks without a mailbox. How Mektup compares with AgentMail.