Ever try to add a new SMTP alias to a user account, only to be told that the alias was already in use? Finding where this alias is already in use becomes really easy by performing a custom search inside of Active Directory.

From AD Users & Computers, right click on your domain and choose Find.
From the Find: drop-down list, select Custom Search
Click on the Advanced tab
Enter the following LDAP query:

proxyaddresses=SMTP:alias@domain.com

Remember to change alias@domain.com to the email address that you are attempting to find.