+ - remove mail_deliver_session after all, do all the stuff transparently
+ by hooking into mailbox_copy().
+ - use this hook also to do the mail deduplication: 1) sort all destination
+ users, 2) create mail_user only once for each user, 3) remember in
+ src_mail the previously copied mail, 4) use that for mailbox_copy()ing
+ to following recipients
- notify_sync() could have "what changed" struct with old/new flags
- mailbox deletion could really be "rmdir", which fails if everything isn't
already expunged. IMAP DELETE could fail with "mailbox changed during