2 seperate domains (more, but for simplicity's sake...)
1 E2k server.
1 SMTP relay for each domain.
So on the Exchange box are all the mailboxes. email@example.com and firstname.lastname@example.org. B.COM mail comes into mail.b.com and A.COm mail comes into mail.a.com. Then the relays forward the mail (after spam filters and other fancy archiving stuff) to the Exchange box and voila, we have delivery. When both a.com and b.com users send mail, it is routed to a smart host - only one - so b.com's email actually goes OUT on a.com's SMTP relay. That's bad and i don't want that and legally I can't have that. I can add virtual host all day long and IPs to the Exchange box as necessary. So how can I route outgoing mail from email@example.com's email to mail.a.com and firstname.lastname@example.org to mail.b.com?