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openmail automatically groups related emails into threads using standard RFC 2822 email headers.

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How it works

When an email arrives or is sent, we resolve which thread it belongs to:
  1. Check In-Reply-To header — If it matches an existing message’s Message-ID, the email joins that thread.
  2. Check References header — If any value matches an existing message’s Message-ID, the email joins that thread.
  3. No match — A new thread is created.

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Outbound replies

When you send with a threadId, we automatically set:
  • In-Reply-To → the Message-ID of the last message in the thread
  • References → all Message-ID values from the thread
This ensures your reply threads correctly in the recipient’s email client (Gmail, Outlook, etc.).

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Thread context

Fetch all messages in a thread via GET /v1/threads/:id/messages. Messages are returned in chronological order, giving your agent the full conversation history to inform its next action.