Stop threading by subject

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(This page was written for Thunderbird but also applies to SeaMonkey.)

Messages can be grouped into threads based on common Subjects, and/or by information stored in the References/In-Reply-To headers. There are many e-mail clients that do not add In-Reply-To and References headers to messages, which is the typical way to determine a thread. Threading based on the subject can be annoying when you get unrelated messages with common subjects such as "Hi!" or "Report!". By default Thunderbird tries to groups messages into threads in the following order:

  • Common reference in the References header
  • Common reference in the In-Reply-To header
  • Same Subject, and the Subject contains Re:
  • Same Subject
  • Check if the message is an ancestor of an already-threaded message.

There are several settings that manage threading that can be changed using the Config Editor at Tools -> Options -> Advanced -> General.

  • mail.thread_without_re will thread based on the subject even if there is no Re: in the subject. It defaults to false starting with Thunderbird 3.0.
  • mail.strict_threading disables threading based on the Subject and only uses the References/In-Reply-To headers headers to determine threads. It defaults to true starting with Thunderbird 3.0.
  • mail.correct_threading will thread messages correctly using the References/In-Reply-To headers regardless of the order the messages were added to the folder. It defaults to true starting with Thunderbird 3.0.
  • mailnews.localizedRe defines a comma-delimited list of alternative prefixes to "Re:"

If you want to stop all threading by Subject set mail.strict_threading true.

See also

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