Take a look here: http://forum.avast.com/index.php?topic=10428.0 to see how to set up secure email with avast!.

Since SSL/TLS e-mail is encrypted and decrypted in the client, external virus scanners (including avast!) can’t read or scan it. The solution is to pass e-mail in and out un-encrypted from your client (Outlook Express, Thunderbird, …) to a proxy program (Stunnel) that does the actual ssl or tls encryption/decryption of the pop3/smtp e-mail and communicates directly with the ISP server on the appropriate ports. Download here: http://www.stunnel.org/download/binaries.html