re-routing e-mails

I am trying to set up exchange server to re-route specific e-mail address to a forwarding address I have set up these specific accounts with internet mail forwarding and it works to a point. Example if I send an e-mail to one of them from our system using there old e-mail address it is forwarded on to the new e-mail address .
The problem is if an external e-mail is sent to them it is then not forwarded on to the new e-mail address.

Any help please would be greatly appreciated

pjs