redirect SMTP port (linux) Postfix mail
iptables -t nat -A PREROUTING -d 10.1.1.1 -p tcp --dport 26 -j REDIRECT --to-ports 25 just you need to change the ip to the server ip, i have tried it my self and working fine, References http://www.5dollarwhitebox.org/wiki/index.php/Howtos_Basic_IPTables#Port_Redirects