Something I ran into on a system of a relative.

When running AVG 8′s outbound mail scanner and your ISP does not require you to provide a username and password when connecting for an outbound SMTP session…

Make sure that you do NOT check the checkbox in the SMTP settings dialog near the username and password field in ThunderBird. If you do, you probably have trouble sending email.

This seems to only happen with AVG 8 or later when you check the scan outgoing messages option in AVG system tray application.