Many broadband suppliers block other suppliers email SMTP servers. If a user connects to your hotspot and wish to send and receive email,you may want to redirect SMTP requests to a specific IP which is supported by your ISP.
This option is available in DD-WRT. Login to the router’s admin page and go to Services > Hotspot, and enable SMTP Redirection.
It is also possible to set this as a firewall rule:
‘/usr/sbin/iptables -t nat -A PREROUTING -p tcp –dport 25 -j DNAT –to YOUR_SMTP_IP_ADDRESS’
Replace YOUR_SMTP_IP_ADDRESS with your SMTP server’s IP address.