What we are currently looking at doing is forwarding emails from as user at our sister company to his new email address here.
The IT people at our sister company have created a contact and added his new email address in there as an smtp. Then on his user properties under 'Delivery Options' added the contact to forward to.
This is all working fine and his he getting emails from our sister company and us.
What seems to be happening at the moment is when he creates or replies to a message from our sister company he gets the following bounce backs:-
'The message could not be delivered because the recipient's destination email system is unknown or invalid. Please check the address and try again, or contact your system administrator to verify connectivity to the email system of the recipient.'