Maybe you have already read these threads as you participate on them:
http://forum.avast.com/index.php?board=2;action=display;threadid=2095
http://forum.avast.com/index.php?board=2;action=display;threadid=6400
Can a tweak on avast4.ini file help?
See ‘Settings’ on my signature (section [MailScanner]) 