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Threaded replies now possible in Email Workers

Mar 12, 2025, 06:00 PM

Email Routing

Due to increased demand for email use in AI Agents and other forms of task automation, we changed the rules and limits of how Email Workers can reply to incoming emails.

It's now possible to keep a threaded email conversation with an Email Worker script as long as:

• The incoming email has to have valid DMARC ↗.
• The email can only be replied to once in the same EmailMessage event.
• The recipient in the reply must match the incoming sender.
• The outgoing sender domain must match the same domain that received the email.
• Every time an email passes through Email Routing or another MTA, an entry is added to the References list. We stop accepting replies to emails with more than 100 References entries to prevent abuse or accidental loops.

Here's an example of a Worker responding to Emails using a Workers AI model:

AI model responding to emails

import PostalMime from "postal-mime";
import {createMimeMessage} from "mimetext"
import { EmailMessage } from "cloudflare:email";

export default {
  async email(message, env, ctx) {
    const email = await PostalMime.parse(message.raw)
    const res = await env.AI.run('@cf/meta/llama-2-7b-chat-fp16', {
      messages: [{
        role: "user",
        content: email.text ?? ''
      }]
    })

    // message-id is generated by mimetext
    const response = createMimeMessage()
    response.setHeader("In-Reply-To", message.headers.get("Message-ID")!);
    response.setSender("agent@example.com");
    response.setRecipient(message.from);
    response.setSubject("Llama response");
    response.addMessage({
      contentType: 'text/plain',
      data: res instanceof ReadableStream ? await new Response(res).text() : res.response!
    })

    const replyMessage = new EmailMessage("<email>", message.from, response.asRaw());
    await message.reply(replyMessage)
  }
} satisfies ExportedHandler<Env>;

See Reply to emails from Workers for more information.