Hi - I'm happy to help with your Mac issue. Please remember to Accept when we are completely finished and you are satisfied with my answer.
Is your email also from Comcast? When you look at your account settings in Entourage, what is showing for Sending mail? What is the entry for SMTP Server? When you clock on the Advanced Button just underneath the Send Mail Section, is it set for SMTP? What is the Port #, and is it set for authentication?