I'm not talking about forwarding because then he would never receive email from me or his secretary on the cell. It's a direct replacement for the "reply to" address.
I receive and handle all business email sent to abc@def.com and it is my job to pass it on after printing and archiving it. My husband's cell uses xyz@rrr.com for email. He does not receive abc@def.com since a) there is a lot of spam etc that he doesn't have time to handle and b) he has been known to delete email on the cell in such a way as to (accidentally) delete it from the server and prevent me from archiving it 
To see what I am referring to, in Windows Mail click tools>accounts>(click on an account)>properties, the first tab (general) has a "reply address" box to fill in. What then happens is that he uses his cell (xyz@rrr.com) to email someone. When that person hits Reply, it goes to this reply address (abc@def.com) we had set up instead of the cell he first used. That way I, as his intermediary, filter out emails and forward him what he needs to see. And he can't delete email accidentally.
Since I check both his email accounts, it's probably not that important. But he wants to make it easier to archive by having all replies sent to the one reply-account.