Either
(1) Change your Outgoing Server security setting in Thunderbird from "TLS" to "TLS, if available" or to "SSL". From the menu select "Tools", "Account Settings...", scroll to the bottom and select "Outgoing Server (SMTP)", then select "smtp.gmail.com" and click the "Edit..." button. If you select "TLS, if available" then the Avast Mail Shield will be able to scan your outbound mail. If you select "SSL", Avast won't scan your outbound mail but it won't prevent it from being sent, either.
or (2) Disable the Mail Shield. If Thunderbird is using SSL to retrieve your inbound mail, the Mail Shield isn't scanning your inbound mail anyway. Open the Avast user interface, click "Real-Time Shields" and then "Mail Shield", click the "Stop" button and select "Stop permanently". (If you'd prefer that the Mail Shield scan your inbound mail, let the Mail Shield run and in Thunderbird change "Use secure connection:" for your gmail account from "SSL" to "Never".)
As for you, you Three Stooges of AV evangelism, is installing SP3 the best solution your fuzzy brains could produce? If you've forgotten your infamous update that flagged every file as a virus before rendering the OS inoperable by repairing the "infestation", I assure you that we haven't. From a user's point of view your Mail Shield is a mad leap of faith - disable the known network security provided by open-source Thunderbird and trust that closed-source Avast is somehow covering the gap. You aren't simply scanning traffic and passing it on - you're rewriting packets for no better reason than to force your users to use your Mail Shield. I'm not sure whether dropping the STARTTLS capability from the mail server's EHLO response was strategic or simply a bug. I'm inclined to think it a bug, since none of you three seem to have a clue why TLS but not SSL was affected.
