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Email Thread-ID Explainer

Threading is a client-side decision based on Message-ID, In-Reply-To, References, and (Gmail) Subject. This page explains all four.

The four signals clients use to thread
  1. Message-ID — every message has a unique one. Replies reference it.
  2. In-Reply-To — header on a reply that contains the parent Message-ID.
  3. References — chain of all ancestor Message-IDs. The key signal for threading depth.
  4. Subject (Gmail-specific) — Gmail also groups by normalized subject (strips Re:, Fwd:, etc.). Outlook does NOT.
Common breakage patterns
  • ESP rewrites Message-ID: The reply's In-Reply-To no longer matches anything in the recipient's thread.
  • Outlook desktop forwards without preserving References: The chain breaks; next reply starts a new thread.
  • Subject change in Gmail: Changing the subject by more than "Re:" can fork the thread in Gmail.
  • Bot/notification senders setting fresh Message-IDs: Each notification appears as a new thread instead of stacking.
What it does

Walks through the four signals each major client uses, with examples of how a missing or rewritten header breaks threading.

When to use

Diagnosing why a reply landed as a new thread. Building any email-sending system that needs threads to stay intact.

FAQ

Why does Outlook sometimes break Gmail's threads?+

Outlook (old desktop) rewrites Message-ID on forwards and breaks the References chain. The reply lands as a new thread for the next-hop client.

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Nate SummersCo-Founder, Outsolvi

Nate built Outsolvi after watching every email-tracking tool he had ever used lie to him about opens. Outsolvi runs Tier 1 to 5 confidence scoring on every open, native in Outlook and Gmail, so the number on the dashboard is one a rep can actually act on.

Last reviewed May 25, 2026Editorially independent

We update these pages when the underlying mechanics change — new mailbox-provider rules, new tracker behavior, new measurement gaps. The dates above are real revisions, not auto-touches.

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