Migrating Exchange Users

Newbie to Domino trying to migrate my Exchange 2K users via Domino Administrator. Everything is fine until I click the Register All button then I get a bomb saying somthing to the effect “The internet address for this user is not mm/dd/yy” I then look at the logs which says “The internet address for this user is not unique” This server has only ONE user on it, as it is a new server. I don’t know how to proceed, Thank you in advance for your help.James