Software

The problem is that you mail cannot be routed to a user not located on your server i.e. you are trying to send mail to an external address.

As you would not want to allow routing outbound to any address (spam relay) you can overcome this problem by creating custom recipients for those external addresses that you would wish to use. However, this is not practical for a whole load of addresses.

Your best bet would be to use Outlook in c/w mode and/or a VPN.

David