Sorry, I should be more specific.
Maybe you can add the following line to the [MailScanner] section of \data\avast4.ini:

Log=20

Then generate some traffic, simulate the problem (i.e. force the avast mail scanner to time out by sending an email with attachment) and then post here the contents of the file \data\log\aswMaiSv.log

Details: http://forum.avast.com/index.php?topic=12234.msg103474#msg103474