TonyH
I am having exactly the same problem with Avast 4 Home that I downloaded and installed today. I have completely removed Norton but I do run MailwashPro.
Did you make a typo? In one respose you say you don't and now your saying you do use MailWasher Pro.?
Since MWP checks the mail first, I need the original settings assigned to you by your ISP. Enabeling bounce in MWP isn't recommended because it only increases the amount of spam.