You may have many special rules. Each rule contains the following information.
This is the destination domain you want to control.
This is the host which will receive (and retransmit)
the messages for the destination domain. The forwarder
can be a fully qualified name or a UUCP name depending
on the mailer used to reach that forwarder.
This rule may apply to a destination domain and all machines/subdomains under it. Note that you may select this option and still provide another special routing for one host/subdomain of this domain. This rule will take precedence over this option.
You pick the mechanism which must be used to reach the forwarder.