dovecot - Sep 2005 - ALL TCP/IP connection REST when one mail contains format error

Hi,

I have setup a dovecot pop3 server on IP_A and using mbox format
I have 6 users behind a NATed network, the public IP is IP_B.

And one mail of one user seems contain format error.
He always cannot receive the 5th mail from anywhere.

Then when he receive mail in the intranet, all the TCP/IP connection REST 
between IP_A and IP_B.  http/ssh/pop/smtp etc. all NG.

After I delete the 5th mail from webmail, then the services work well.

1.It seems unreasonable, Can dovecot improve it and let it donot influence 
other users and other service?

2.What may cause mbox format error? and how can we avoid it?

Best regards,