Solving “e-mail address already exists in this organization” using LDAP query in ADUC

Using the Windows 2003 Active Directory Users and Computers utility, right click on the domain, choose Find. In the Find: dropdown list, choose “Custom Search” and in the In: dropdown list, choose “Entire Directory”. Click the Advanced property page and enter the following LDAP query (in my example, at least):

proxyaddresses=smtp:user@domain.com

Click Find and you should find the object that already has that SMTP address.