If you don't want to mess up with Apache and other mail services on alternative port esecially on dynamic hosting,then there is a easy way out:
Issue this command:
iptables -t nat -A PREROUTING -i eth0 -p tcp --dport $srcPortNumber -j REDIRECT --to-port $dstPortNumber
So if you want to redirect port 81 to Apache:
iptables -t nat -A PREROUTING -i eth0 -p tcp --dport 81 -j REDIRECT --to-port 80
So you can set up alternative port in your Dynamic DNS provider and host your own server.
