Controlling HTTP & SMTP IP flow for 3 NIC's

[Tim Alberts]

A little background first..I run 2 servers, mail and web.  The mail 
server is down for hardware problems.  I'm running both email and web on 
one server.  The web server has 3 network interfaces 1 for public email, 
1 for public web, and 1 for private network.  I use 3 cards because the 
router I connect to the internet won't recognize multiple IP's for a 
single hardware MAC.

The problem is, that my email messages seem to be going out the web 
network interface.  This is typically not a problem except for the 
reverse DNS lookup  fails which at least one domain (Comcast.Net) rejects.

My question therefore is, how can I route my sendmail traffic to go out 
the correct ethernet interface?  I'm sure I'll need to do the same for 
the web traffic so web site spoofing alarms are triggered.

I have told sendmail to listen to the correct interfaces, but that 
apparently doesn't mean only write to those interfaces.  I don't see how 
to control this by setting up my routes and I can only really think of 
ways to block it in IPtables, not re-route it.