I don't know what to suggest, despite requesting what data you entered in the avast SMTP
What settings did you put into the avast SMTP server address ?
We ask questions so we can get a better understanding of the problem might be (incorrect settings, incompatible port number, etc.) without information we are working in the dark.
I don't need to know your user name and password, just that they are required and they have been entered and the checkbox ticked, but without the other information I can't even guess as to the cause of the problem. See example of my setup.