OK, here goes.
The new 4.5 was suppossed to handle the configuration by itself so that you would not need to use the 127.0.0.1 internal server setting,
BUT...it does not work. I went through the same thing tonight with my Thunderbird AND Pegasus.
It was necessary for me to re-run the MPW and have Avast set things back to the old way using 127.0.0.1 and reconfiguring the username format to add the "#pop.isp.com and #smtp.isp.com endings.
Both email clients work now and the "insert" nows appears in scanned email messages both ways.
Outlook Express is still having problems, but I suspect that it is the same thing.
Manually reconfigure the settings. If you use the MPW and check Remove all protection, then Avast basically disconnects itself and your email is no longer scanned (even though it wasn't anyway...

)
I will notify the Avast Team that there are still some bugs in the release.
Let me know if this helps you.