Fix wrong hostname suggestion box.box.example.com

FIX: If you use me@box.example.com (as advised by the script) as your mail address, the default primary hostname will be box.box.example.com. 

sed already strips the local part of $EMAIL_ADDR to advise a PRIMARY_HOSTNAME so I appended another sed-script that strips "box." if it is in the beginning of $EMAIL_ADDR after the local part is gone.