I have experienced the same issue, at least twice. (On one occasion it occured after I made significant changes to my mail folders from Outlook, resulting in a "confused" Hero).
On one occasion, simply deleting the mail account on the Hero, then recreating it resolved the problem without further complications. However on another occasion I also had problems getting the Hero to properly recreate the account, have Exchange accept my credentials (username/password) and sync.
Try deleting the mail account on the Hero and then rebooting the Hero prior to recreating the account. I have discovered a few problems like this which are resolved after rebooting the phone.
Additionally, verify that the Server Name is saved correctly in the Hero. I found that it persisted in corrupting the server name being pulled by the email address, until I manually corrected the Server Name, and sync'd right up...
So: 1. Reboot the phone, 2. Recreate Mail Account..verifying accuracy of all paramaters/settings including the server name...
Good luck..