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?
Gmail Conversations – How Gmail Decides to Thread Email Messages
gmailgmail-conversations
Best Answer
The following conditions must be met:
test
andre: test
will work; buttest
andtest 123
won't).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.