How to search AD for particular email address.

What you will want to do is open up Active Directory Users and Computers and right-click the domain and select Search. Select the drop-down arrow in the Find field to select Custom Search. If you have multiple domains make sure to select Entire Directory on the In field. Now just click on the Advanced tab and put the following text in the LDAP Query - proxyaddresses=smtp:. Now all you have to do is click on Find Now and if the email is in use it will show the user account that is using it.