From time to time I’ve had to figure out which user account has a specific email address. Actually its more like finding who has the “firstname.lastname@example.org” so another “more senior” person can get it. Well if you work in a smaller company this can be kind of easy…but if your directory has thousands of accounts it becomes more difficult and time consuming.
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:<whatever the email is you’re looking for>. 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.