Windows SMTP Virtual Server - Smarthost Backup

Okay, here is the dilemna

I am running Windows 2003 Server Web Edition.
I am sending all mail to a smarthost in the DMZ.

I know there is the option to send without using the smarthost, but that is not possible with the network setup. We are not allowing outbound email to public network, only to dmz smarthost.

My question, How do I obtain redundancy? Can I specify two smarthosts in the smarthost box separated by a comma. Or can I put a DNS name in the smarthost box and then put 2 DNS A records into the host file and it will do round robin? Or does the smarthost dns lookup only do A records or does it actually lookup MX records for the DNS name specified in the smarthost box, in which I could put two mx records for the smarthost specified...

Any ideas??
Thanks in advance..