I had a simliar problem … it turned out to be the security setting … avast has problems with STARTTLS

imap.mail.yahoo.com --------- port 465 … security SSL/TLS Full Success

mail.weblinksonline.co.uk — smtp port 465 … security START/TLS No Avail
change to security SSL/TLS Full Success
mail.weblinksonline.co.uk — smtp port 465 … security SSL/TLS Full Success

smtp.live.com ----------------- smtp port 465 … security START/TLS No Avail
change to security SSL/TLS No Avail
change smtp port to 25 and update Avast mail settingsNo Avail
removed port 25 from avast smtp mail settingsNo Avail
change change to security back to START/TLS avast mail settings Full Success
smtp.live.com ----------------- smtp port 25 … security START/TLS (port 25 not declared in Avast mail smtp settings) Full Success

Conclusion
Avast mail scanner does not work with security START/TLS
smtp.live.com needs security START/TLS
Set security to SSL/TLS and if the server does not work with that then set its SMTP port to 25 and do not declare the port in Avast smtp mail settings

*edit typo *