What is the difference between a message-ID and a thread-ID?
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A Message-ID is a unique identifier for a single email. Every email that leaves a mail server gets its own Message-ID, generated automatically. It looks like this: <20231215143022.abc123@example.com>. The receiving server uses it to prevent duplicate delivery (if the same email arrives twice, the Message-ID tells the server to ignore the second copy).
A Thread-ID (technically called In-Reply-To and References headers) links related emails together into a conversation thread. When you reply to an email, your mail client copies the original email's Message-ID into the reply's In-Reply-To header. That's how Gmail, Outlook, and Apple Mail know which emails belong in the same conversation view.
Here's the practical difference: Message-ID is for the servers (prevents duplicates, tracks delivery). Thread-ID headers are for the humans (groups emails into conversations in your inbox).
As an email sender, you don't need to do anything about Message-IDs. Your ESP or mail server generates them automatically. Thread-IDs only matter if you're sending reply-chain emails (customer support threads, Gmail's "On [date], you wrote..." style). Most marketing email doesn't use threading because each campaign is standalone.
One edge case worth knowing: some ESPs reuse Message-IDs across sends (bad practice, causes delivery issues). If you're seeing duplicate suppression problems or missing emails, that's worth checking. But honestly, if you're not running a custom mail server or building a support ticket system, you'll never need to think about these headers.
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