[Dovecot] time out error and email with attachments

[Roberto Cipollini]

I'm facing some problem with emails and attachments.
I'm not sure it's dealing with MTU (it is set to the maximum number 
before a fragmentation occurs).

I have no errors from pop3, but the following message is the best I can 
show you just to understand my problem.
The log shows this:

"imap(roberto.cipollini at allmediasolution.com): Oct 04 17:35:03 Error: 
Timeout while waiting for release of exclusive fcntl() lock for index 
file /mail/allmediasolution.com/roberto.cipollini/.INBOX/.imap.index "

I think that the pop3 process is locking the index file (thunderbird was 
  waiting to download the file) and the imap (i tried to get the file 
from webmail) is going time out.

Can anyone tell me how to fix the time out?
any suggestion?
do you need more information/logs?
can I try different setting in dovecot conf?

thanks for any kind of help.
Roberto