Email – If port 25 is blocked

If your isp blocks port 25 (it should)
redirect traffic coming in on port 2525 to port 25
/sbin/iptables -t nat -I PREROUTING -p tcp --dport 2525 -j REDIRECT --to-port 25
# make it so that this command runs on server boot up.
if ! grep -qai 2525 /etc/rc.local; then
echo '/sbin/iptables -t nat -I PREROUTING -p tcp --dport 2525 -j
REDIRECT --to-port 25' >> /etc/rc.local
fi