I have a working exchange 2007 system that hosts several accounts such as firstname.lastname@example.org, and email@example.com. I have other users on a unix mail system, firstname.lastname@example.org and email@example.com. Exchange will not let me send email from username1 or username2 to either username3 or username4. It appears that exchange tries to do local delivery for username3 and username4, but obviously doesn't find them. How do I configure exchange to deliver to users it hosts, and then pass on any other firstname.lastname@example.org emails to the unix host and let it try to deliver them.