My problem with mail is that it keep telling me it's rejecting the password.
It's only started since I installed 10.4 (I have the 10.4.1 upgrade).
I only carried out the upgrade type install, not a full delete and install.
This ticket has been moved to the open forums for more exposure.
Please follow this link:
http://www.macosx.com/forums/showthread.php?t=224068