I’m not 100% sure, but I think that Gmail uses SSL (Secure Socket Layer) connection. avast! doesn’t support SSL, and Vlk said, most likely it never will…
Google GMail Settings
The Google GMail service offers email client access for retrieving and sending emails through your Gmail account. However, for security reasons, GMail uses POP3 over an SSL connection, so make sure your email client supports encrypted SSL connections.
Google Gmail Incoming Mail Server (POP3) - pop.gmail.com (SSL enabled, port 995)
Outgoing Mail Server - use the SMTP mail server address provided by your local ISP
So, in short, the avast mail scanner doesn’t support SSL (it has never had).
interception of the traffic by avast
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 \data\avast4.ini, and to the [MailScanner] section, add the following line: