You go to create an email address and get the error “This e-mail address already exists in this organization”
so you search and find http://support.microsoft.com/kb/280765 and it does not help at all.
The Solution :
Active Directory Users and Computers utility
Right click on the domain, choose Find.
In the Find dropdown list, choose “Custom Search” and “Entire Directory”.
Click the Advanced property page and enter the following LDAP query
Click Find and you should find the object that already has that SMTP address.
Now you know what object and probably don’t have a problem if you are really in a bind right click, exchange tasks “Remove Exchange Attributes”