When inserting an envelope in a thread and its Message-ID already exists
with an associated envelope, overwrite the association if the previous
associated envelope is from a foreign mailbox and current envelope is
not. This happens when mail from a sent folder has been inserted in eg
your INBOX, but somehow INBOX has a copy of your own message as well.
This can happen when mailing lists that send you copies of your own
posts.
The problem with this was that in IMAP your mailing list copy was unseen
and you could not mark it seen because the thread only knew about your
Sent mailbox copy.
Manos Pitsidianakis