Hi,
you get an erron while you're sending a mail, can you show to us the Thunderbird SMTP configuration?
Have you tried to change in avast mail configuration from SSL to TLS?
All your accounts (gmail & hotmail) have this problems?
Are you sure you weren't receive correctly the mail by other people?
I've gmail and avast free 7.0.1426 and TB11 and I've no problems...
Any other security program installed?And previous was removed completely?