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
[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 ~]# 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! 
Postfix working now.
Postfix working now.