Find existing SMTP address in Exchange

In complex environments consisting of several AD structures, SMTP duplicates can occur.

Perhaps a firm was acquired by another and agreed to keep both infrastructures in place only to find over time that duplicates have not been identified in another forest domain.

Have you ever tried adding an SMTP address in exchange only to find that another account already has it listed as an SMTP address?

Well you can find which accounts have duplicate smtp addresses by the following LDAP query in ADUC and this method has saved me time and time again:

1: Open Active Directory Users and Computers

2: In the Find list, select Custom Search.
3: In the Domain list drop-down, select entire directory
4: Click on the Advanced tab and enter: 

ProxyAddresses=SMTP:john.smith@domain.com 

5: Click on Find now button and it will bring the object that contains that particular SMTP address.

NB: This works for all versions of Exchange (even Exchange 2010)