When you use the Active Directory Users and Computers MMC snap-in to add or change an e-mail address, no notification of a duplicate e-mail address is generated, so Maildsmx.dll (Exchange Extension) cannot check for uniqueness.

NOTE: You only receive the This e-mail address already exists in this organization warning when the management console notifies Maildsmx.dll that an e-mail address has changed.

To workaround this behavior, use the Find feature in Active Directory to make sure that the e-mail address you want to specify is not used by any other object.