How to maintain mail conversion (reply / forward / reply to all like gmail) of email using Python pop/imap lib?

I've develop webmail client for any mail server. I want to implement message conversion for it — for example same emails fwd/reply/reply2all should be shown together like gmail does...

My question is: what's the key to find those emails which are either reply/fwd or related to the original 开发者_如何学Cmail....

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The In-Reply-To header of the child should have the value of the Message-Id header of the parent(s).

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Google just seems to chain messages based on the subject line (so does Apple Mail by the way.)