Recently a customer came to me with a problem. One of his users was missing the E-mail Addresses tab on the user object in Active Directory.
The user had been sending and receiving email for months without a problem, and the other Exchange tabs in AD Users and Computers (Exchange General, Exchange Features, and Exchange Advanced) were present. Here's an example:
This happens because the Exchange Alias is missing and the Exchange Recipient Update Service (RUS) cannot update the email addresses. The fix for this is simple -- enter an Alias for the user on the Exchange General tab. Once you do this, the E-mail Addresses tab becomes visible, as shown below.