avast can’t check encrypted email, as it is effectively ‘outside’ the email client and encryption process. That is the whole point of encryption so it doesn’t handle it well if you standard pop3/smtp ports for encrypted email.
avast intercepts the outbound email to scan it and once done then hands it off but it believes it is plain old smtp port 25 traffic so doesn’t know to initiate the ‘STARTTLS’ command. Sorry this is a very simplified interpretation of my limited understanding of this. Also see http://forum.avast.com/index.php?topic=26654.msg217996#msg217996
What ports are you using and what does your ISP (?)/email service say you should use ?
There are ports allocated for SSL/TLS connections. Secure Ports POP3 995; SMTP Ports, Thunderbird-587 - Outlook/Express-465. Also see http://email.about.com/od/gmailtips/qt/et010605.htm