I have strange problem. In last Friday I had a problem with dovecot. It didn't see directories with mailboxes in /var/vmail. Somehow it crashed because probably two things (a lot of this type of lines in mail.err file):

Nov 23 07:05:02 s1 dovecot: lda(poli@poli.li): Error: User initialization failed: Initializing mail storage from mail_location setting failed: mkdir(/var/vmail/poli.li/poli/Maildir) failed: Permission denied (euid=5000(vmail) egid=5000(vmail) missing +w perm: /var/vmail, dir owned by 0:0 mode=0755)

and maybe

Nov 12 09:27:59 s1 dovecot: imap(artur@firmastanaszek.pl): Error: Corrupted index cache file /var/vmail/firmastanaszek.pl/artur/Maildir/dovecot.index.cache: Broken MIME parts for mail UID 559 in mailbox INBOX: Cached MIME parts don't match message during parsing: Cached header size mismatch (parts=)
Nov 12 09:27:59 s1 dovecot: imap(artur@firmastanaszek.pl): Error: unlink(/var/vmail/firmastanaszek.pl/artur/Maildir/dovecot.index.cache) failed: No such file or directory (in mail-cache.c:28)

and after this event probably all mailboxes download again all emails from 1.11 to 22.11. They were duplicated in mail clients.

Pozdrawiam / Best Regards

Piotr Bracha