mail alias problem

Hi all, well i am configuring backuppc for awhile, anyway my current problem is, i set a smarthost, configured the mail to use our ISP smtp server, and its working successfully thanks to ALLAH, now i want to set an alias for mails( it was working when it was using the local smtp server), so the backuppc send mail to backuppc, while my /etc/aliases have backuppc: myexternalmail, but its not working now with the external mail, simply i want backuppc when it mail backuppc its converted to myexternalmail@mydomain.com, is it possible ? and how ?, thanks so much.