Delivering Email to Disabled Accounts

When using MS Exchange, mail sent to a disabled account bounces. If you need to allow mail to be delivered to the inactive user's mailbox, you will need to grant SELF the Associated External Account permission. This is probably the best solution, if you only have to manage a small number of mailboxes or if you sometimes choose to bounce the mail. If your policy is to always accept the mail, you may want to look into the hotfix that MS recently released.

Take a look at KB 319047 for information on setting the required permissions for SELF or KB 903158 for information on the hotfix.