[Dovecot] Corrupted Mail?

[Arthur Dent]

Hello All,

I have a Dovecot v2.2.12 installation running on a Fedora 20 box which
is the mailserver for my family. This means that it is serving only 4
accounts.

I use fetchmail->procmail to feed Dovecot and I read my mail on clients
such as Evolution on (another) Fedora 20 machine, iPad/iPhone or
squirrelmail on the web (running on the same Fedora 20 server).

The average mail throughput for the whole family is around a couple of
hundred messages per day, but each (and every) day I get between 3 and
10 of the following messages:  

dovecot: imap(mark): Error: Next message unexpectedly corrupted in mbox
file /home/mark/mail/MLists/Fail2Ban at 3407473: 1 Time(s)

I have googled, but with little success. What I have found seemed to
suggest that removing the .index files and allowing Dovecot to rebuild
them will solve this. I did:
find /home -type f -name "dovecot.index.*" -exec rm -f {} \;
but it has made no difference.

Can anyone help me solve this please?

Thanks in advance

Mark