Exchange – Find if email address is already assigned

If you need to find out if an email address (SMTP address) has been assigned to a user or public folder, use these steps:

1. Open Active directory users and computers
2. Right click on domain and click find,
3. Change the search option from “Users, Contact, and Groups to “Custom Search”
4. Click on the “Advanced” tab,
5. In the field enter:
   proxyAddresses=smtp:email@yourdomain.com
6. If the address is in use, the user / public folder will appear in the result field.

Steps from Kazmarek