So you went to add a new address to a user account in Active Directory, and you got an error that this address already exists? Sucks huh? Especially when you don’t know where it could be? Here’s one way to track it down.
On your domain root in AD Users and Computers, right click, select Find. Select Custom Search, click the Advanced tab, and enter an LDAP query like so:
proxyaddresses=smtp:emailaddress@youwanttofind.com
(I am assuming you are not a complete NEWB and you know you should replace that email with the one you want.)
Then click the Find Now button. You should see a in the results below any object that may have this address on it.