Threads are sequences of e-mail messages which directly or indirectly refer to each other. How is this information specified when the messages are transported.


Each message has a "Message-ID:" header. Replies have an "In-Reply-To:" header with the Message-ID of the previous message. Messages in a thread also often have a "References:" header with a list of the Message-ID for the whole chain from the message to the first message in the thread. If this chain becomes very long, it is sometimes truncated in the middle.

Keeping the "Subject:" intact, except for additions of "Re: ", is also sometimes used to indicate the thread.

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