iptables:

LOCALNET=192.168.1.0/24
MAILSERVER=87.250.251.37

iptables -A FORWARD -p tcp -s $LOCALNET -d $MAILSERVER --dport 25 -j ACCEPT
iptables -A FORWARD -p tcp -s $MAILSERVER -d $LOCALNET --sport 25 -j ACCEPT
iptables -A FORWARD -p tcp -s $LOCALNET -d MAILSERVER --dport 110 -j ACCEPT
iptables -A FORWARD -p tcp -s MAILSERVER -d $LOCALNET --sport 110 -j ACCEPT

iptables -t nat -A POSTROUTING -p tcp -s $LOCALNET --dport 110 -j MASQUERADE
iptables -t nat -A POSTROUTING -p tcp -s $LOCALNET --dport 25 -j MASQUERADE

Примерно так