>From many replies I understand manual thread-joining and -breaking exists with mutt's manual commands and default subject breaking -as Gmail does- would not be preferred, while not only version control systems vary subjects within a thread, but also discussions with slight off-topic forks and therefore slightly changed subjects should stay in the same thread.

The reason why I asked my question in the first place is because I have lots of mail-discussions going on between about 10 board-members who are not able to meet in real life often enough to decide everything by conference, so our mailboxes pile up with suggestions, remarks with longer growing thread-histories and evolving addressee-lists. Those addressee-lists vary by individual choice, often without confirmation of other participants to involve some new addressee's, sometimes resulting in leakage.

I thought to revive mail2forum, a plug-in for phpBB, to force people to use existing addressee-lists per 'circle' and archive all e-maildiscussions in a forum, so people wouldn't be e-mailed for every subject and could lose/drop their own mail. Threads should be compressed to keep mostly only original messages - if available -, and small citations, or links to original texts if needed. This thread-compression is functionality the existing mail2forum doesn't have, so that's where for example notmuch comes in.

Discussing around I understand that phpBB misses the very basic feature of thread-forking: Every slightly off-topic remark in a phpBB-message can only be splitted to a completely new thread.

I wonder whether that is blocking for my own situation, as participants in our discussions don't change the subject-line very often, but it could probably affect the viability of mail2forum as an open source-project.

I don't see how I can easily manually manipulate threads with a mail client when mail2forum automatically reads and processes new incoming mail, so my efforts with notmuch will probably stick to the 'optional' subject-splitting-solution.

As, however, mail2forum should handle postponed e-mail as well (and exchange former quotes with their original texts), probably patching from a manually altered maildir wouldn't be such a big step. I however haven't studied all that's needed there yet.

By the way, before I spend too much time on mail2forum - does anyone know an (open source?) group-mail project with user authentication to centrally stored messages that already does have satisfying thread-compression?