If your sending mail is on Port 25 and is SSL (encrypted) (even though the error reads No on SSL), Avast by itself can not scan the mail and you could just disable the Internet Mail scan. Encyrpted mail can not be unencrypted by Avast, you could use additional programs, such as stunnel, to do that but the future Avast 5 is suppossed to have this feature without additional 3rd party programs.
Also, you may have a message that is exceeding the timeout of the server - try to up OE timeout to 600 (5 min), restart OE and see if it sends then.