[Dovecot] Can't donwload mails with some attacchments

[Lethux]

Hi.
Does anyone find the same problem with attachments bigger than few Ks?
The client (thunderbird) waits for the file to be downloaded. It can be 
waiting for several minuts and tryies before having it from dovecot.
Can it be something against the lock of the file, the processes or 
something else?
Any workaround or bugfix in the stable/alpha rel?
thanks.
L.