The issue is not that ...
I use Avast on my desktop system (running XP). When the dialog comes up, click on the second tab of the dialog. There you will see an option to remember the choice for this certificate - click that checkbox, then click on Accept. You'll have to do the same thing for both the incoming and outgoing servers, so be aware that even though you've accepted the certificate (and told it to remember) for the incoming server you'll get another dialog the first time you send mail to that account. And of course if you have more than one account, that's twice for each account ...
