Avast’s Mail Shield would normally intercept emails so they can be scanned before they are sent or actually received.

Do you have any SSL/TLS (secure) email connections ?

Whilst this is a standard SMTP port 25 connection avast may be trying to send it as a secure connection, but I don’t see anything in your error information to support that. Try this and see if it helps AvastUI > Settings > Active Protection > Mail Shield > Customize > SSL Scanning - uncheck the Scan SSL connections.

Though why this should still be happening even after disabling the Mail Shield is somewhat strange, enable the Mail Shield again and try the above.