Resolution
fact
Directory and Resource Administrator 6.x
fact
Directory and Resource Administrator 7.x
fact
Directory and Resource Administrator 8.0
symptom
The error above may occur when manually modifying or adding an SMTP address to a user account's mailbox properties.
symptom
Error: The proxy is already assigned to another user while updating a User's email address.
cause
The common cause for this error is when the manual changes or newly added address conflicts with an address already is use by another user account.
fix
To determine which user account has the same email address, use the naming convention principles in place for the Exchange environment to locate the other user accounts that could be using the same address. Modify the address for one of the conflicting accounts so that it is no longer the same.