This was a case where whenever we would try to give a specific Email Address to an existing user using "Exchange tasks, Establish Email Address" , it errors out with the below message :

"This email address already exists in this Organization"
ID no: c10312e7

Article 280765 is the solution:

Error message: This e-mail address already exists in this organization
http://support.microsoft.com/default.aspx?scid=kb;EN-US;280765

Method 1 did not find it, so next was to use a Network Monitor capture.

We used the following instructions to disable LDAP Encryption on the server:
http://blogs.technet.com/b/benw/archive/2008/12/15/disabling-ldap-encryption-and-signing-for-netmon-on-an-exchange-server.aspx

We then got our capture, but the question was how to find the string in the capture?

NMSimpleSearch to the rescue! (http://code.msdn.microsoft.com/NmSimpleSearch)

I loaded this expert into Network Monitor, and then used it (from the Experts menu) to search for the email address we were trying to define, and found the search request for the address and then what object was returned.

From these, I could see both the object returned and could see the IP address of the GC we were finding it on.

We used ADSIEdit to connect to the GC port (3268) of the DC shown by the IP address in the capture, which was a child domain DC/GC in the same site as the Exchange server, and we found the object causing the problem, which was a Contact in the same OU as the user we were trying to enable.

This contact did not exist on the DC from the domain the object was from.

So, that meant our problem was replication to this specific GC.

We decided the easiest way to resolve the problem was to remove the GC role from the problem DC, then put it back.

We removed GC role, waited for events in the Directory Services event log indicating it was removed, then added the GC role back. We then verified with ADSIEdit the bogus contact object was gone, and then we successfully mail-enabled the user. using the desired email address