Deb, definetivelly this is not avast trouble. Avast works with as many servers as you want. I use four differents servers. The problem remains on authentication. Some servers need user@server and others just user, for instance.
The server you select while using setup is the default one, used to send files to Alwil for tests, and other things. It’s explained in FAQ on my signature.
I recommend more tests with the email account settings 8)