Open Active Directory Users and Computers:
If you keep your coprorate users in Users go there,
if you have a separate container for corp users, go there.
Even better, if you have a container for Contacts go there...
-right click, New> Contact
Create the new contact. When creating the email account be sure his display name is unique (i.e. can't match his domain account. Click Next
Check Create Exchange Address, then under the space allocated form Email click the Modify Button. Select SMTP, click okay and put the users personal mail address. Click OK. Then Click Next Click finish.
Boom you created an outside contact.
Now find his Domain account. Open it. Click the Exchange General Tab. Click the Delivery Options Button. Select Forwarding Address, Select Foward To, Click Modify. Enter the Unique Name of the Contact you created for this person. Click the Deliver messages to both forwarding address and mailbox.
Click Ok. Done
Now, he will not be able to send as someone@corp.com. So I'd check periodically to find out what he's getting there. I'd also set a 30 day limit on this forwarding service.
Also, I swear there used to be a way to hide the contact from the GAL. I'd hide is contact and his domain accounts.