avast ignores SSL (encrypted) connections, that is the whole purpose of SSL to keep prying eyes out.
The problem occurs when the standard port 25 (smtp) is used for secure communication, avast monitors that port and is expecting standard smtp non-encrypted traffic.
So does your email server use port 25 for SSL secure email ?
What is the full text of the message or post a screenshot of the error message ?
The standard port for SMTP secure sending of email is 465, for POP3 it is 995, you shouldn't add these ports to the avast Internet Mail's redirected ports.