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?
How does Gmail decide to thread email messages?
gmail;gmail conversations
The following conditions must be met:The subject must be similar (e.g. test and re: test will work; but test and test 123 won't).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.