SMTP: Configure a mail transfer agent (MTA) to accept inbound email from other systems.

In order to test a mail server, you will need to set up a DNS server.

Remove the sendmail package (sendmail is more complicated to configure):

# yum erase sendmail

Install the postfix package:

# yum install -y postfix

Add a new rule to the firewall:

# iptables -I INPUT -m state --state NEW -m tcp -p tcp --dport 25 -j ACCEPT

Save the firewall configuration:

# service iptables save

Activate the postfix service at boot:

# chkconfig postfix on

Start the postfix service:

# service postfix start

Let’s assume that your server is called mail.example.com on the 192.168.1.0/24 network.
Edit the /etc/postfix/main.cf file and change the following directives:

myhostname = mail.example.com
mydomain = example.com
myorigin = $mydomain
inet_interfaces = all
mydestination = $mydomain, $myhostname, localhost.$mydomain, localhost
mynetworks = 192.168.1.0/24, 127.0.0.0/8

Check the syntax:

# postfix check

Check the non-default configuration:

# postconf -n

Reload the postfix configuration:

# service postfix reload

Set the ‘allow_postfix_local_write_mail_spool‘ SElinux boolean to ‘on’:

# setsebool -P allow_postfix_local_write_mail_spool on

Test from a client (it should display: 25/tcp open smtp):

# yum install -y nmap
# nmap mail.example.com

Alternatively, test from a client:

# yum install -y telnet
# telnet mail.example.com 25

In addition, you can look at configuring a MTA to forward (relay) email through a smart host.