Mail Forwarding - SmarterTools

One of our clients forwards one email address to an external mailbox.

With smartermail rather than just forwarding the email it resend's the the message TO the new address FROM the original sender - this is playing up with our anti spam software as neither domain are in its local list.

Is there either A) a way of setting a separate outbound route for a hosted domain / mailbox or B) a better way of forwarding these emails ?