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Sometimes I'll notice false positives (includes an email that isn't part of the thread) and false negatives (misses an email that should have been part of the thread). How does Gmail decide which messages should and shouldn't be part of a thread?

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don't blame Gmail, it's the best mail-threader I always have seen ;-) –  neo Jul 1 '10 at 18:41
    
I've never seen false positives, I'm surprised that those even exist. –  mbillard Aug 1 '10 at 11:45

3 Answers 3

You can find more info about that here:

http://taint.org/wk/GmailThreadingDetails

and here

http://xkahn.zoned.net/software/evolution/threads/

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Please don't post links to answers without providing any content. That does not help anybody find the answers here in the future. –  Robert Cartaino Jul 1 '10 at 22:04

The short answer: the subject line.

Google states:

a conversation will break off into a new thread if the subject line of the conversation is changed, or if the conversation reaches over 100 messages.

More info from Google

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Yes, that is what also says in the second link I provided. I though it would be good to also give some background on the whole matter :) –  feniix Jul 1 '10 at 19:49
    
I've seen it break between 60 and 80, so I'm not sure how reliable that article is. But it's close. –  dgw Aug 3 '10 at 3:03
up vote 20 down vote accepted

The following conditions must be met:

  1. The subject must be similar (e.g. test and re: test will work; but test and test 123 won't).
  2. The sender must be a part of the thread OR the in-reply-to header must be used.

The in-reply-to header can be used via Gmail's interface by simply replying to the thread. This is what enables forwarded messages to be a part of the same thread even though the sender is different.

If you want more information, you can check out my blog post where I posted more detailed information about my findings.

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