Entourage, like many Microsoft products, is full of bugs
That's no secrect.
I want to ditch Entourage. I want to clear all the junk out of MAIL and start over.
The first thing you need to do is be sure you have a backup copy of all you mail from your Mail accounts. Best way to do that is to copy the entire Home/Library/Mail folder to your Desktop or an external drive of some sort, and then you can delete the Home/Library/Mail folder (i.e., just delete the Mail folder in that path), but don't empty the Trash. Then, quit Mail and Entourage if either are running, go to Home/Kibrary/Preferences and delete the com.apple.mail.plist file; but don't empty the Trash.
Once you've done that, then you can restart Mail and setup your accounts and account preferences from scratch. Make sure you know your usernames and passwords before you start doing this.