That could be the problem, avast currently can't scan SSL encrypted email. If that SSL email is sent using the standard port 25 then avast will me monitoring that port and try to scan it and then send it, but it can't hand off the secure connection so it fails.
Secure email normally uses different ports than the standard unencrypted SMTP protocol, for Thunderbird-587 (not 25), what happens then as avast only monitors ports 25, 110, 119 and 143, the email would be sent without avast interaction.
However there are some ISPs that insist on using port 25, what is your ISP ?
Do they have an alternative port to use for SSL/TLS email ?
Because you are sending outside of your system then avast isn't scanning outbound email, when it comes in it would be received as normal unencrypted email on port 110. If it wer SSL/TLS incoming it would probably suffer the same fate as sending from your system.
So it isn't odd that you can't send from your system, because you are using SSL. If you unchecked the SSL bit then I'm sure that it would get sent, but your ISP might reject the unsecured email connection.
