Relay-restriction mechanisms seem to be based on a message's source without regard to its destination. Can you set up Exchange 2000 Server relay restrictions to allow relay only to specific domains but from any inbound host?
No, you can't configure relay restrictions in that way. When you're configuring your server, you can tell it which domains to accept mail for by setting recipient policies that control which SMTP addresses Exchange assigns to recipients. These policies specify a form of address (e.g., FirstName.LastName@yourdomain.com), and the Recipient Update Service (RUS) uses the policies to add SMTP addresses to each addressable object. (Of course, if you don't set up a DNS MX record for each domain you want, you won't receive mail for those domains.) However, Exchange 2000's relay-control mechanisms let you control only which machines send mail to you, under the assumption that you'll control where the mail goes. You can write a transport event sink that implements this functionality.