Avast doesn’t support SSL mail (nor do other AV scanners). It is encrypted from the client (OE) to the server and Avast can’t read it. Turn off either SMTP scanning in Avast! or SSL in OE.
How come you’re using SSL SMTP over port 25 (it’s usually 465)? Are the port details in OE set up correctly?
Or maybe it’s a negotiated SSL version (the “STARTTLS” feature)…
OE uses TLS and labels it SSL on port 25. Maybe it actuallys uses SSL if you change the port to 465?? Thunderbird has separate boxes and uses SSL on port 465 and TLS on port 25. Sure confuses me.