All Tools
Free ToolReference

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.

You know the rule. See where you sit against it.

Reference pages tell you the benchmark. Outsolvi tells you whether your team is above or below it. per-rep, per-deal, per-week. and which behavior changes move the number. From $7/user/mo yearly.

See your number
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.

Keep going

Articles, glossary entries, and other tools on the same topic.

Free tool
Open Rate Checker
Estimates what share of your recorded opens were machines rather than people, then shows the human-read rate underneath.
Open
Article
Email tracking in Gmail: what really works
Gmail has no built-in read receipts for normal accounts. How tracking works in Gmail, the image-proxy trap tha…
Open
Glossary
Apple Mail Privacy Protection
Apple Mail Privacy Protection (MPP) is a privacy feature that launched with iOS 15 in September 2021. When ena…
Open
Feature
Automated Follow-Up
Automated follow-up lets you build a multi-step sequence (send an email now, wait two days, send a follow-up, wait three more, send another) and have Outsolvi send each step for you, on schedule, from your connected Outlook or Gmail account.
Open
Use case
Sales Prospecting
Someone searching sales prospecting email tracking is an AE or sales operations lead trying to figure out whic…
Open
Industry
SaaS
Someone searching email tracking for SaaS is typically an AE, sales-ops lead, or RevOps manager at a B2B SaaS …
Open

Other free tools

Reference
RBL Explainer
Reference
Spamhaus ZEN
Reference
Google FBL
Reference
SSL vs TLS
Reference
STARTTLS
Reference
Email Attribution
Reference
Weekly Checklist
Reference
Reply Benchmarks
Reference
Open Benchmarks
See all 100 tools
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.

About the authorMethodologyArticlesFree tools