How to install and configure SMTP (Postfix) In Red Hat Enterprise Linux 7

The Simple Mail Transfer Protocol (SMTP) is a communication protocol for electronic mail transmission. The most commonly used implementations of SMTP in most Linux distributed OS are Sendmail and Postfix.

Postfix is an open source mail-transfer agent that was originally developed as an alternative to Sendmail and is usually set up as the default mail server. Let's start with Postfix.

Step:1 To verify the /etc/hosts file.

# cat /etc/hosts
127.0.0.1   localhost localhost.localdomain
::1     ip6-localhost ip6-loopback
192.168.122.144	mail.lab.munshibari.biz	mail

Step:2 To install Postfix rpm's or packages.

# yum install -y postfix

Step:3 To configure the Postfix as required.

# vi /etc/postfix/main.cf
::::::::::::: CUT SOME OUTPUT ::::::::::::: 
queue_directory = /var/spool/postfix
command_directory = /usr/sbin
daemon_directory = /usr/libexec/postfix
data_directory = /var/lib/postfix
mail_owner = postfix
# Hostnames normally have prefixe Server
myhostname = loadbalancer.lab.munshibari.biz
# The domain that is actually handling mail
mydomain = lab.munshibari.biz
inet_interfaces = localhost , $myhostname
inet_protocols = all
mydestination = $myhostname, localhost.$mydomain, localhost, $mydomain
unknown_local_recipient_reject_code = 550
#  The list of trusted remote SMTP servers that can relay through the server
mynetworks_style = subnet
mynetworks = 192.168.122.0/24, 127.0.0.0/8
alias_maps = hash:/etc/aliases
alias_database = hash:/etc/aliases
 
  
debug_peer_level = 2
debugger_command =
	 PATH=/bin:/usr/bin:/usr/local/bin:/usr/X11R6/bin
	 ddd $daemon_directory/$process_name $process_id & sleep 5
sendmail_path = /usr/sbin/sendmail.postfix
newaliases_path = /usr/bin/newaliases.postfix
mailq_path = /usr/bin/mailq.postfix
setgid_group = postdrop
html_directory = no
manpage_directory = /usr/share/man
sample_directory = /usr/share/doc/postfix-2.10.1/samples
readme_directory = /usr/share/doc/postfix-2.10.1/README_FILES

::::::::::::: CUT SOME OUTPUT ::::::::::::: 

Step:4 If Firewalld service is running, Need to open the ports in the OS firewall using following firewall-cmd commands.

# firewall-cmd --add-port=25/tcp --permanent
# firewall-cmd --reload

Step:5 To start the Postfix service.

# systemctl restart postfix
# systemctl enable postfix

Step:5 To verify the Postfix (SMTP) service from the client.

# telnet 192.168.122.144 25
Trying 192.168.122.144...
Connected to 192.168.122.144.
Escape character is '^]'.
220 loadbalancer.lab.munshibari.biz ESMTP Postfix
HELO  lab.munshibari.biz
250 loadbalancer.lab.munshibari.biz
MAIL FROM: root@lab.munshibari.biz
250 2.1.0 Ok
RCPT TO: atower@lab.munshibari.biz
250 2.1.5 Ok
DATA
354 End data with <CR><LF>.<CR><LF>
Subject: This is a test message 

Hello,

This is a test message
.
250 2.0.0 Ok: queued as A611340B196
quit
221 2.0.0 Bye
Connection closed by foreign host.

To check email from Postfix (SMTP) server by login with users.

# yum install mailx
# su - atower
$ mail
Heirloom Mail version 12.5 7/5/10.  Type ? for help.
"/var/spool/mail/atower": 1 message 1 new
>N  1 root@lab.munshibari.  Mon Jul 27 14:49  13/444   "This is a test message"
&
Message  1:
From root@lab.munshibari.biz  Mon Jul 27 14:49:33 2019
Return-Path: <root@lab.munshibari.biz>
X-Original-To: atower@lab.munshibari.biz
Delivered-To: atower@lab.munshibari.biz
Subject: This is a test message 
Status: R

Hello,

This is a test message
& 

This is a basic smtp email server configuration and will post some advanced configuration as well.