Search for Email Alias in Active Directory
You are trying to add a new SMTP alias to a user account, only to be told that the alias was already in use.
You don’t know where it is 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: