Many of the exchange Administrators might already be aware of this but for those that are not aware here is my blog post.
If a request comes to you that please forward the emails from a particular mailbox to outside company email address.
All of us will simply create a contact object in exchange & will use forwarding option in exchange management console & will point it to
that contact object.
There is one more way to achieve this & this will avoid using contact objects (that can clutter the system if you are a big exchange organization).
use TragetAddress attribute to achieve this.
Open adsiedit.msc & modify that user object for which you want to add forwarding , add the email address in targetaddress field.
If you have any doubts , do ask me.