1. Run Active Directory Users and Computers.
2. Right-click the domain object and choose Find from the context menu. This will bring up the Find Users,
Contacts, and Groups window.
3. In the Find drop-down list, choose Custom Search.
4. Click the Advanced tab and in the Enter LDAP query: field, enter the following text: proxyaddresses=smtp:emailaddress@”yourdomain.com”
5. Click the Find Now button.
source:
http://www.msexchange.org/articles/Finding-Duplicate-SMTP-Addresses.html
