compA - eth0 192.168.0.1
compB - eth0 192.168.0.2
compC - eth0 192.168.0.3

compB# socat tcp-listen:80,reuseaddr,fork tcp:192.168.0.3:80

iptables -A PREROUTING -d my.public.ip.address/32 -p tcp -m tcp --dport 587 -j DNAT --to-destination a.different.ip.address:587