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)

/var/vmail is not owned by vmail:vmail, which I think it should be in your configuration.

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.

Unlikely that this causes all mailboxes to download all mails.

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Pozdrawiam / Best Regards

Piotr Bracha

Also which version of dovecot is this?

Aki