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Email Threading with Message-ID

Vitor Capretz

Every email carries a Message-ID header, the unique identifier that mail clients and servers use to recognize a message and group conversations into threads.

Starting today, we're including message_id in all email webhooks and in the retrieve and list email endpoints. This has been one of our most requested features, and it unlocks new use cases without any new primitives.

What you can build with it

  • Threaded replies: pass the message_id as the In-Reply-To and References headers on your next send, so Gmail, Outlook, and Apple Mail group it into the same conversation.
  • Reply matching: when an inbound reply arrives, its headers reference the original Message-ID, so you can link the reply back to the exact email that started the thread.
  • Cross-system correlation: reconcile emails sent through Resend with mail server logs, support tools, or your own database using an identifier the whole email ecosystem understands.
  • Deliverability investigations: when working with a recipient's mail admin, the Message-ID is how they find the message on their side.

Included in every email webhook

All email.* webhook events now include message_id in the payload. This includes email.sent, which fires as soon as the email is accepted for delivery, so you can capture the Message-ID immediately.

{
  "type": "email.sent",
  "created_at": "2026-07-08T23:41:12.126Z",
  "data": {
    "email_id": "56761188-7520-42d8-8898-ff6fc54ce618",
    "message_id": "<111-222-333@email.example.com>",
    "created_at": "2026-07-08T23:41:11.894719+00:00",
    "from": "Acme <onboarding@resend.dev>",
    "to": ["delivered@resend.dev"],
    "subject": "Sending this example",
    "tags": {
      "category": "confirm_email"
    }
  }
}

Included when retrieving emails

The GET /emails/:email_id endpoint now returns message_id for every sent email, so you can look it up at any time, not just when a webhook fires. The GET /emails list endpoint returns it for every email in the page too, so you can backfill Message-IDs in bulk.

{
  "object": "email",
  "id": "4ef9a417-02e9-4d39-ad75-9611e0fcc33c",
  "message_id": "<111-222-333@email.example.com>",
  "to": ["delivered@resend.dev"],
  "from": "Acme <onboarding@resend.dev>",
  "created_at": "2026-04-03 22:13:42.674981+00",
  "subject": "Hello World",
  "html": "Congrats on sending your <strong>first email</strong>!",
  "text": null,
  "bcc": [],
  "cc": [],
  "reply_to": [],
  "last_event": "delivered",
  "scheduled_at": null
}

Conclusion

Threading, reply matching, and correlation are the foundation of building conversations on top of email. Exposing the Message-ID is the first step, and we're working on more primitives to make replies even easier.

If you have any questions, please reach out to us and we'll be happy to help.