There are more elegant ways to do this, but this is decent and doesn't require much typing
port forward some port on the firewall machine to the internalmachine's sshd (2022 in this example). then do this on the remote machine:
ssh -X -p 2022 email@example.com
That should do it. The earlier poster was right too. You really shouldn't have people logging into the firewall, even if it is just to jump to another box.
And yes, I read the part about not wanting to enable SSH on the Redhat box. I'm ignoring it though until you present a good reason to use rsh over ssh ... rsh was great back when the 5.25 floppy had lots of storage and wheels were square, but there's little reason to use it now ...