---
title: "Message-ID — Glossary — Mbox Viewer"
description: "A globally unique identifier assigned to each email message, specified in the Message-ID header. It is used to track messages, build conversation threads, and detect duplicates when merging archives."
url: https://mboxviewerpro.com/glossary/message-id/
language: en
source: Mbox Viewer
---

[Mbox Viewer](https://mboxviewerpro.com/)  [Glossary](https://mboxviewerpro.com/glossary/)  Message-ID

# Message-ID

A globally unique identifier assigned to each email message, specified in the Message-ID header. It is used to track messages, build conversation threads, and detect duplicates when merging archives.

The Message-ID header (RFC 5322 section 3.6.4) contains a string that is intended to be unique across all email ever sent, typically formatted as a local-part@domain string such as <CABcde12345@mail.gmail.com>. The sending mail server generates this identifier at the moment of transmission. Replies include the original message's Message-ID in their In-Reply-To and References headers to link the conversation.

Message-ID is the primary key used by threading algorithms to reconstruct conversations. It is also used during deduplication: when merging two MBOX files that may overlap — for example, two Google Takeout exports from different dates — comparing Message-IDs allows the application to identify and skip messages that already exist in the target archive.

In rare cases, Message-IDs may be missing (from very old messages) or duplicated (from buggy sending software). A robust archive tool handles these edge cases by falling back to heuristic matching on other headers such as Date, From, and Subject when a Message-ID is absent or unreliable.

## Related terms

[

In-Reply-To / References

Email headers (In-Reply-To and References) that link a reply to the message it responds to, enabling mail clients and archive tools to group related messages into conversation threads.

](https://mboxviewerpro.com/glossary/in-reply-to/)[

Deduplication

The process of detecting and removing duplicate email messages from an archive, typically by comparing Message-ID values, to avoid redundancy when merging multiple MBOX files.

](https://mboxviewerpro.com/glossary/deduplication/)[

Threading (JWZ)

The process of grouping related email messages into conversations by following In-Reply-To and References header links, typically using the JWZ algorithm that supports up to four levels of nesting.

](https://mboxviewerpro.com/glossary/threading/)

[Back to the full glossary](https://mboxviewerpro.com/glossary/)

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