I would like to setup a Linux Router with Fedora. Here is what I have and would like to do...

eth0 - T1 12.x.x.0/24 network .1 is gateway
eth1 - T1 63.x.x.0/25 network .1 is gateway
eth2 - DSL 151.x.x.77 single static IP (Modem in bridge mode)
eth3 - LAN 192.168.x.x network

Email filter device is on the LAN at 192.168.x.3. I need to provide SNAT to that device from both T1 interfaces...
12.x.x.44 -> 192.168.x.3
63.x.x.44 -> 192.168.x.3

All outgoing traffic will route through DSL except for traffic destined for addresses on either eth0 or eth1 (web sites).

I assume I would need the following:
route add -net 12.x.x.0 netmask 255.255.255.0 gw 12.x.x.1
route add -net 63.x.x.0 netmask 255.255.255.128 gw 63.x.x.1
route add default gw 151.x.x.77

I also assume I would need to setup iptables with FORWARDs for the 192.x.x.x networks to eth3.

What I am not sure how to do, is properly setup the incoming redirections so that the email filter can be access from both eth0 and eth1 by different IP addresses.

Any/All help with this is greatly appreciated.

I think you need to use DNAT, because what comes into 12.x.x.44 should be sent to 192.168.x.3. Also allow forward between interfaces.
iptables -t nat -A PREROUTING -p tcp -d 12.x.x.44 --dport xx -j DNAT --to-destination 192.168.x.3