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.
- Message-ID. every message has a unique one. Replies reference it.
- In-Reply-To. header on a reply that contains the parent Message-ID.
- References. chain of all ancestor Message-IDs. The key signal for threading depth.
- Subject (Gmail-specific). Gmail also groups by normalized subject (strips Re:, Fwd:, etc.). Outlook does NOT.
- 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.
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See your numberWalks through the four signals each major client uses, with examples of how a missing or rewritten header breaks threading.
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 built Outsolvi after watching every email-tracking tool he had ever used lie to him about opens. Outsolvi grades every open as a person or a machine, in Outlook and Gmail, so the number on the dashboard is one a rep can actually act on.
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.