I haven't used it myself, but ..
If you're interested in the client part of mailing, perhaps create a dummy account, join the dovecot and postfix mailing lists, create two new filter rules that apply either "postfix" or "dovecot" labels and archives (= unapplies "Inbox" label) messages depending on the to/cc line, and try it out :-).
Hmm. Wasn't it about showing entire threads of messages in INBOX?
Yes, or under any other label than "Inbox".
Also, the messages does not need to have the "Inbox" label to show up under Inbox - they can have only the "Sent" label, and they will still show up in the thread as long as they're part of it as per In-Reply-To: or whatever.
Obviously useful since if you send an item ("Sent" label applied) and receive an answer ("Inbox" label applied), the exact same conversation will show up under both labels, and with the full list of messages relevant to it.
With Dovecot it would be done by:
mailboxes = INBOX "Sent Messages" search = INTHREAD REFERENCES MAILBOX INBOX
So it matches entire threads of messages in INBOX.
So if I understand correctly, this will basically tell the IMAP client *all* the messages relating to a given conversation, regardless of whether they were sent or incoming?
(With the flip-side that software which expects to use different folders, eg say a Horde webmail alternative for the account, will stop working against this mailbox... Or is this all or none regarding which mailbox are affected?)
INTHREAD would need adding support for INTHREAD extension.
Ok, what does that do? (Apologies for being in learning mode.)