I'm helping a user who, while using her Notes 6.0.2 client with her Notes 6.0 template local mail file, gets the "Illegal Circular Use: CoreEmailClasses" error when opening mail. She can click this error a dozen times or so, and it eventually goes away. This doesn't happen on her server mail file, only her local one.
The folders of the mail file need to be upgraded also. The old folder design uses a different script library that is no longer available. Look at the section on "Upgrading to the Notes 6 folder design" in the Admin Help database.