Postfix is an open-source Mail Transport Agent (MTA), which supports protocols like LDAP, SMTP AUTH (SASL), and TLS. It routes and delivers emails.

You will need to uninstall the sendmail from the server before installing the postfix because sendmail is the default MTA in Redhat/CentOS.
[root@server ~]# yum remove sendmail

Requirement -

i. The server should have correct MX records. 

ii.  Firewall  should be disabled. You can use below command to disable the firewall -

[root@server ~]# service iptables stop

[root@server ~]# service ip6tables stop

[root@server ~]# chkconfig iptables off

[root@server ~]# chkconfig ip6tables off

Installation

Use below command to install postfix - 

[root@server ~]# yum install postfix

Configuration

Open the postfix config file -
/etc/postfix/main.cf

Find the below lines and edit them as shown below -
[root@server ~]# vi /etc/postfix/main.cf

myhostname = your server hostname

mydomain = your domain name

myorigin = $your domain name 

inet_interfaces = all  

mydestination = $myhostname, localhost.$mydomain, localhost, $mydomain 

mynetworks =  add your network range

home_mailbox = Maildir/ 

Start the postfix service - 
[root@server ~]# service postfix start

Starting postfix:                                          [  OK  ]

[root@server ~]# chkconfig postfix on

Testing the postfix -

[root@server ~]# telnet localhost smtp

Check Mail -

Navigate to the user mail directory and check for the new mail - 
[root@server ~]# cd /home/user1/Maildir/new/

[root@server new]# ls

After 'ls' command, you'll get the details of email with date, time, to and from. 

All done!