Recently I became unable to send messages using Thunderbird, but I could with Mutt (imap). I finally found the culprit and just wanted to share if it was affecting anyone else. The changes to main.cf were as follows:
- smtpd_recipient_restrictions = permit_mynetworks permit_sasl_authenticated reject_unauth_destination + smtpd_recipient_restrictions = permit_mynetworks, permit_sasl_authenticated, reject_unauth_destination
