Routing internet mail out through a different firewall

I hope that this makes some sense.

We would like to route some SMTP mail out through a different firewall 
than all of other internet mail now travels through. The Domino server is 
running in another partition on the iSeries, where we have 4 other Domino 
servers running. I have tried using the Relay Host field and the Smart 
Host field on the Server Doc, and that didn't work. I tried a Connection 
Doc and that didn't work. I am thinking that this is a network (DNS?) 
issue on the iSeries, since my simple tests didn't work. If that is the 
case, then I won't be able to use the iSeries Domino server to do this, 
since the other partitions will need to use the real outbound firewall to 
send mail to the internet.

If you have some ideas, I would appreciate it.

Thanks.

Gregg