1. Update Avast via GUI → Maintenance → Update Program.

  2. In Thunderbird account settings disable SSL/TLS and set the server to use standard ports.

  3. Open Avast, Mail Shield → Settings → Expert Settings → SSL accounts and delete all accounts.

  4. Reboot.

Avast will determine the ssl-accounts automatically after reboot.

If that does not help the sending problem, please come back here. We will then have to configure Avast manually according to the required server settings.

We will help you with that.