ipchains and redirection

[aphro@portal.aphroland.org]

hi

i have a firewall machine on a network somewhere and it has
2 real ips and 1 fake ip. our mail server acts as a secondary
MX incase the primary MX is down. here's the current
setup:

123.456.789 - primary ip addy of the firewall on 'net
123.456.788 - secondary ip (also resolves to our mail
server)

port forwarding is setup so 123.456.788 port 25 to forward
to the internal mail server. however outgoing traffic goes
through 123.456.789 which for some reason screws with
some of the spam sites like rbl or whatever. so what i'm
wondering is if it's possible with ipchains or something
to redirect all outgoing traffic on 123.456.789 port 25
to 123.456.788 port 25. i know -j REDIRECT but it seems
to only be able to specify a port not an ip. i am thinking
of trying to do -j REDIRECT in combonation with rinetd
to redirect out of the other interface but i've never
used rinetd in this fashon, and am pretty sure it won't
work. maybe i can use -j REDIRECT in combo with -i eth1:1
to do what i need? i will have to play around with it..

any ideas are welcome :)

thanks!!

nate