Esetleg valami ilyesmi?
iptables -P FORWARD REJECT
iptables -F FORWARD
iptables -t filter -A FORWARD -i eth0.1 -p tcp --dport 25 -j REJECT
iptables -t filter -A FORWARD -m state --state NEW,ESTABLISHED,RELATED -j ACCEPT
iptables -t nat -F
iptables -A POSTROUTING -t nat -o eth0 -s 10.0.0.0/24 -j MASQUERADE