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.
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