De acordo com este tópico intitulado: IMAP Dovecot Error - Corrompido cache de índice 10.6.4 parece que você acabou de precisa fazer o seguinte:
Scribit
Re: IMAP Dovecot Error - Corrupted index cache 10.6.4
Nov 30, 2010 11:10 AM (in response to Scribit)
I am not sure if this is the best procedure and there may be unintended consequences, but this is what I did to resolve the issue.
I stopped the mail service.
From a shell, I navigated to each directory where an issue was reported.
In these directories, I renamed the following files, prepending them with "old."
dovecot.index dovecot.index.cache dovecot.index.log
EXAMPLE:
mv dovecot.index old.dovecot.index
I then restarted the mail service. These 3 files were recreated for each IMAP folder on client access.