Recently I was cleaning up some Exchange mail-enabled contacts and moving them to different OU’s.  I had to create a new contact and kept getting an error for a duplicate email address.  I couldn’t find the duplicate using the normal search methods, so after some digging I found this site that provided a custom LDAP query: [more]

http://www.danlance.co.uk/index.cfm/2007/11/14/How-to-find-the-user--entity-an-email-address-is-associated-with-in-Exchange-2003

Basically you search for attribute “(proxyAddresses=smtp:user@domain.com)” to give you the object with this email address.