dovecot - Oct 2013 - retr errors

My ISP uses Dovecot and I have had an ongoing problem
for a while using several email clients.

Sometimes the response to a retr request is mal-formed. The expected 
response
"+OK nnn octets" is not returned. The response looks like it
started somewhere in the message headers.

Sometimes a retry can clear the
problem but I usually need to delete the first message via a putty session.

The problem, when present, is always on the first message. Never seen
it in the middle of a series of messages. This problem has been seen on
different machines, different versions of Windows, at home and on the road.

I have wireshark traces showing good and bad sessions....
=====================stat
+OK 93 1000437
retr 1
+OK 6946 octets
Return-path: .....
========================retr 1
  of blahblah at blah.com designates 2607:f8b0:4001:c03::235
  as permitted sender) smtp.mail=blahblah at blah.com;       dkim=pass 
header.i=@blah.com
Reply-To: android-developers at googlegroups.com
Precedence: list
Mailing-list: ......
=================My ISP has been non-helpful. Any ideas how I can track
down the problem?

Thanks
Bill

Hi Bill

You should send the wireshark traces to your ISP and ask him to fix it. 

At least one would require the doveconf -n output and the version of dovecot.
Probably a bug in an older dovecot version?

Regards
Daniel

Hi Bill,

any intercepting virus scanner or personal firewall software between your mail
client and the dovecot server?

Regards
Daniel