Forward port from localhost to external server?

Question

linux

I installed exim4 on a separate server and now I need other servers (debian) to contact this external server when sending mail to localhost:25 192.168.0.2
Desirable rule for iptables
This does not work .0.1 --dport 25 -i eth0 -j DNAT --to-destination 192.168.0.2:25

Answer 1

And by the way, how can you have
-d 127.0.0.1
and
-i eth0
Look at ifconfig and you will understand everything.

Answer 2

put postfix and configure it as relay.