No, that is just one factor.
Your ISP may insist of using SSL (secure encrypted) email in which case avast can't scan it and that doesn't matter what your email client is.
Your complication is using winME where the email client account settings have to be changed to use the avast localhost proxy, using the avast Mail Protection Wizard. Once changed they shouldn't revert back unless you change it back or you run the wizard again and remove the protection (from memory). I doubt you did either so they should stay the same unless you have even more security software that blocks changes.
I can't see how your ISP can change to Gmail
So you will have to explain that one to me.