Directory and Resource Administrator 6.x
Directory and Resource Administrator 7.x
Directory and Resource Administrator 8.0
The error above may occur when manually modifying or adding an SMTP address to a user account's mailbox properties.
Error: The proxy is already assigned to another user while updating a User's email address.
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.
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.