determine why mail clients download mails 2nd time

[Poliman - Serwis]

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 at poli.li <poli at poli.li>): Error: User
initialization failed: Initializing mail storage from mail_location setting
failed: mkdir(/var/vmail/poli.li/poli/Maildir
<http://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 at firmastanaszek.pl
<artur at firmastanaszek.pl>): Error: Corrupted index cache file
/var/vmail/firmastanaszek.pl/artur/Maildir/dovecot.index.cache
<http://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 at firmastanaszek.pl <artur at firmastanaszek.pl>): Error:
unlink(/var/vmail/firmastanaszek.pl/artur/Maildir/dovecot.index.cache
<http://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*
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