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[SLUG] routing problem

Hey sluggers,
I have a strange routing problem atm and am wanting a few pointers.
Lets say the machine in question has 5 NICs connected to 5 subnets. Also a
modem. Thats 6 interfaces in total.
Now everything seems normal in ifconfig and route. All the routes are there,
the subnet masks are normal etc. Linux box can ping hosts on each of the
connecTed subnets.
A simple ipchains script that just masqurades for 4 of the subnets ( the
private ones) that looks like this:

/sbin/insmod ip_masq_ftp
echo 1 > /proc/sys/net/ipv4/ip_forward
/sbin/ipchains --flush
/sbin/ipchains -A forward -s -j MASQ

Yes yes i know that is not a private address space but lets assume
I can't change it. which i can't :/
So that gives each machine on the network internet access just fine.
Problem is that the 5th subnet is the one the company mail server is on. Its
a public address (an MX record points at it) and its routable from the rest
of the internet fine.
Now, why don't clients on the various private subnets see each other? Why
can't they ping each other?
The default ipchains rules are ACCEPT btw.

Anyone got any ideas?
Is the ip_forward setting the only one you need to route between those
Is there some sort of magic routing i have to turn on to get traffic to flow
between clients?