<snip>
Never (if you aren't guru) edit your iptables by directly editing your config. Always use iptables binary.
It is said there... -t option shouldn't be on that line. The line should be only -A PREROUTING -p tcp --dport 26 -j REDIRECT --to-port 25 because you're already in the *nat section of the config.
But again, you should enter this command instead of editing the config: iptables -A PREROUTING -t nat -p tcp --dport 26 -j REDIRECT --to-port 25
Thanks Tomas and Christopher