How to open port 25 in Centos?

Question

linux

I want to open port 25 (smtp) for Postfix.
Did it like this:

iptables -A INPUT -p tcp --dport 25 -m state --state NEW,ESTABLISHED -j ACCEPT
firewall-cmd --permanent --add-port=25/tcp --zone=public
firewall-cmd --permanent --zone public --add-service smtp
firewall-cmd --reload

Checking from another host:

map 172.21.164.11
...
PORT    STATE  SERVICE
22/tcp  open   ssh
25/tcp  closed smtp
80/tcp  open   http
443/tcp closed https

Why do some ports appear as closed in this list? How are they different from those. what is not displayed?

Answer 1

Probably it would be more correct: one , two , three .

Answer 2

iptables -I INPUT -p tcp --dport 25 -j ACCEPT
service iptables save

Answer 3

A counter, perhaps stupid question, but you need to get an answer to it: Is Postfix running?