The following rules must be met:
- The subject must be similar.
- The sender must be a part of the thread OR in-reply-to must be used.
1) The subjects do not have to be exactly the same, but they must contain a pre-approved prefix (e.g.
re: testwill be in the same thread). I haven't tried all the prefixes, but some of the valid ones are:
FWD:. If you modify the subject in any other way, it will start a new thread (e.g. if you modify
2) The sender of the email message must be a part of the thread, or otherwise it will start a new thread. The one exception is if the in-reply-to header is supplied. You can get the in-reply-to header by replying to the original email through Gmail. For example, let's say you have email@example.com forward messages to firstname.lastname@example.org. Now let's say email@example.com sends you an email to firstname.lastname@example.org. The message will also be delivered to email@example.com. If you use the reply form in your email client (e.g. gmail) from the firstname.lastname@example.org, it will most likely automatically add the in-reply-to header for you. This way even though email@example.com was never a part of the thread that exists on firstname.lastname@example.org, it still goes into the same thread.
One interesting thing to note is that if you send email messages from Gmail they will also be threaded. The rules are exactly the same as when you receive them, except for one minor detail. If you send the same exact message twice with no subject prefix (e.g. subject is
re: test) it does get threaded on the receiving end, but not on sending end. Conversely, if it does contain a prefix (e.g.
re: test) it will be threaded in both cases.
Now that I have a better understanding of how this feature works, I can try to limit the number of false positives and negatives.