problems routing

["Tim" <tim-lists@cimx.com>]

I have a 192.168.100.0 network with my iptables box sitting at
192.168.100.12 and a mailserver at 192.168.100.10 and also with a public ip
and name.

I have several users who are both on the road and work in the office
commonly.

I want to make a rule for them so that when they request mail.cimx.com it
redirects to 192.168.100.10.

I tried to make the following rule:
iptables -t nat -A PREROUTING -p tcp -d $MAIL_IP -j REDIRECT 192.168.100.10
but it seems to not work.

I then changed it to:
iptables -t nat -A PREROUTING -p tcp -d 204.192.102.210 -j LOG
to see the request and I see the request in the logs.

Is there something I am missing?

Thanks in advance.
-- Tim