In the past few weeks, emails were not being received by recipients and then began getting “unable to connect with server” while sending emails via Thunderbird. With trial and error (I have 3 email addresses and sent repeated test messages), it was not Avast VPN, but the antivirus program (version 20.10.2442 - build 20.10.5824.626). By adding Thunderbird to the “list of allowed apps”, was again able to send emails. Same problem occurred on my wife’s computer, again running both VPN and same version of Avast Antivirus. Is the antivirus program no longer scanning incoming and outgoing emails?
Irv
See if anything mentioned here helps you re Thunderbird and Avast: