So I saw this address today when replying to an email after changing a users Primary email address, alias, SPN. User account was being taken over by a new person in this special position.
Odd as it never happened before but after some digging around its because we migrated from Exchange 2003 to 2010 and the user legacy name in AD was using the old mail server address for some odd reason. I will have to figure out why later on.
So I copied the legacy name attribute in AD Users and Computers from a new working user and changed the name on the end to match the special user and now emails can be sent with the new email address.