Most Smart Hosts Look for MX Records First

Specifying a smart host is pretty simple in Exchange, but there are a few tricks that can help you provide the fault tolerance you're looking for.

First, you can specify multiple smart host to spread the load. By doing this Exchange will use each of them in a rotating order so that each smart host will handle and forward email equally. This method provides fault tolerance but if you want to use of one server to handle most of the traffic and fail back to another host if that server is too busy or down you'll need to use some DNS basics.

By design Exchange performs a DNS query on the value specified in the smart-host field to see if there is an MX for it before it moves on to check for the IP via A record. This means you can specify one smart host in Exchange and configure MX records for it to have Exchange route mail a number of different ways.

Configuring them with different MX cost will route all mail to your lowest cost MX record first and if it is "tarpitting" or down for any reason, Exchange will move to the next host.

See http://technet.microsoft.com/en-us/library/cc750376.aspx for more details.