How to open port 25 on the server?

Question

linux

Hello. Mail stopped working the other day.
Began to look. It turns out that the case is in closed ports 25 465 587.
I don’t understand how to open it. I found a couple of manuals, but they did not help. Maybe someone knows a more understandable and simple way to open ports?
Here is one of the manuals vasilisc.com/21-examples-iptables
Linux server debian 7.

Answer 1

If iptables is difficult for you - do not take it for now.
Try ufw (apt-get install ufw)
In it, according to the documentation, open the necessary ports, the documentation will do and this help.ubuntu.ru/wiki/%D1%80%D1%83%D0%BA%D0%BE%D0%B2 ...
For example, open port 22 (ssh) and the ones you need, and then, with the sudo ufw enable command, turn on the firewall.