[Linux] Secondary MX

[Flavored UNIX]

I have two redhat boxen on seperate networks that I'd like to act as
secondary MX's for each other.  I have the DNS part down (if I close SMTP
on one box, mail for it goes to the other), but I can't figure out how to
make the secondary sendmail queue undeliverable mail for its primary -- I
keep getting 551 rejections ('we do not relay').

I've added IP and host names of the machines to each's
"/etc/mail/ip_allow" and "/etc/mail/name_allow" (through linuxconf), as
well as manually adding names to "/etc/mail/access" and then running
"makemap hash /etc/mail/access.db < /etc/mail/access"; still no
success.  (Yes, I restarted sendmail.)

The thing is, I get the feeling this isn't the right track.

What I want is if machine FOO is down, mail sent to FOO goes to machine
QUUX (this part works).  When machine QUUX receives FOO's mail, it should
queue it until it is deliverable, and it should only do this for FOO.  
This part doesn't work: the sender is rejected for unauthorized
relaying.

As I understand it, all the relaying files I mentioned above work on the
sender's address (if the recipient destination is not local), so that's
not where one would configure a secondary MX.  So how does one configure
sendmail to relay based on destination?

Thanks,
Javier