Or rather - I want ACL support definitely, but I'm trying to find a good way to solve this for Maildir mailboxes as well as in general for a multiuser Unix system. :-/
Use POSIX ACLs. Available for Linux from <http://acl.bestbits.at/>.
It would seem to be most in the spirit of Bincimap to not solve this anew when you can just let the OS do it. Otherwise you're starting to go down the road that leads to things like Cyrus where the server is a self-contained system with its own notion of user, permissions, etc. That's fine if that's your goal... but I for one want a good IMAP server that doesn't go that route, and trying to go half way is only going to get you into a quagmire.
:j
