NIS slows incoming email

I have the same issue reported here:

http://community.norton.com/t5/Norton-Internet-Security-Norton/NIS-2012-slowing-down-incoming-e-mail/m-p/622237

The thread is closed so I started this one.

 

The symptoms are similar on 2 machines, one with NIS 2011 on XP and OE as client, the other on Vista/WinMail upgraded to NIS 2012.  On both machines I have Email Protection enabled.  Anti-spam is set on the XP system. 

 

The first mail check after resuming from standby stops on the first message.  Eventually, it restarts and completes.  Subsequent checks seem to run OK. 

 

Symptoms have been around for quite a while - at least from the date of the previous thread.  I eventually did a Wireshark capture which showed that the TCP receive buffer was filling, causing a "ZeroWindow" condition.  In that instance, the transmission resumed after 52 seconds. I don't believe I've ever tweaked the TCP settings on either system, so defaults will apply.  Normal window length shown in TCP packets is 64512.

 

When I disabled Email Protection, there was no delay on the first mail check (in fact it went through very quickly).  I'm assuming this is what's controlling the transmission rather than the email client. My guess would be that the Norton process is freezing and causing the data to back up.

 

Advice in the other thread was to disable Anti-Spam.  I could do the same with Email Protection, but it would be better if it worked.  Any suggestions?