Is your ISP using SSL (secure encrypted) email, the error tends to suggest that is the case ?
avast is unable to scan SSL/TLS email because it is encrypted, avast also can't handle the STARTTLS that is causing the problem.
The problem is more likely that your ISP is using port 25 for secure encrypted traffic when this is the standard port for regular unencrypted SMTP traffic or you have added the secure port to avasts redirect ports.
What port are you using for your SSL/TSL email ?
avast's email scanner is monitoring ports 25, 110, 119 and 143 SMTP, POP3, NEWS AND IMAPI protocols is expecting only these protocols through the respective port, so using one of these standard ports for non-standard purposes is what is causing the problem.