Well usual pop3 mail (incoming mail) mail uses port 110 which avast scans

And usual SMTP mail (outgoing) uses port 25 which avast scans

I just wanted to make sure avast was set up to the right ports, an idea popped into my head that BT yahoo INTERNET mail could use a non usual port (such as localhost).

I take it you have tried temporally disabling/shutting down your firewall, then tried receiving mail with avast open? (to make sure there is not a conflict).

–lee