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Email Threading

Email threading groups related messages into a conversation. Mail clients decide what belongs together from the Message-ID, In-Reply-To, and References headers, and some mail servers additionally assign a thread identifier that EmailEngine exposes as threadId. This page is the entry point; the four pages under it each own one part of the subject.

Quick Start​

Use the reference field of the submit API and EmailEngine sets In-Reply-To and References from the referenced message:

curl -XPOST "https://emailengine.example.com/v1/account/example/submit" \
  -H "Authorization: Bearer <token>" \
  -H "Content-Type: application/json" \
  -d '{
    "reference": {
      "message": "AAAADQAABl0",
      "action": "reply"
    },
    "html": "<p>Your reply</p>"
  }'

That covers replies and forwards to a message EmailEngine can already see. A sequence that starts with no stored message to reference has to carry its own messageId and threading headers; Sending threaded messages walks through one.

Threading Documentation​

1. Threading Overview​

The fundamentals:

  • What Message-ID, In-Reply-To, and References do
  • Why the subject line matters
  • Which backends assign a threadId, and what one looks like per provider
  • Where threadId appears in API responses and webhook payloads

2. Provider-Specific Threading​

How each backend behaves:

  • Gmail over IMAP and the Gmail API: thread IDs and the \All folder
  • Microsoft 365 over the Graph API: conversation IDs and the \All folder
  • Microsoft 365 over IMAP: no thread IDs
  • Yahoo, AOL, and other OBJECTID servers: thread IDs, but no \All folder
  • Other IMAP servers: no thread IDs

3. Searching Thread Messages​

Retrieving every message in a conversation:

  • One search against \All where the backend has it
  • One search per folder where it does not
  • Building a thread from headers when the server assigns no threadId

4. Sending Threaded Messages​

Keeping a sequence you send in one conversation:

  • Setting messageId and extending References with each message
  • When to use reference instead
  • Detecting a Message-ID the receiving server rewrote

See Also​