Conclusion: Gmail uses SSL
So, in short, the avast mail scanner doesn't support SSL (it has never had).
1. interception of the traffic by avast
2. actual scanning of the traffic
Unchecking the SMTP box in Internet Mail provider's settings disables 2 but not 1.
To prevent avast from doing 1 (which makes sense in your case as you're using SSL), please open the file <avast>\data\avast4.ini, and to the [MailScanner] section, add the following line:
StartSMTP=0
Besides using MS Outllook (not Express) and The Bat! for SSL, here is a workaround for SSL and POP3 accounts:
http://forum.avast.com/index.php?board=2;action=display;threadid=8775to use SSL with other clients and also have the protection of avast.