Postfix Mail Server:

In this scenario:
IP address of server = 192.168.1.111
Hostname = el6
Domain Name = wtuto.com

Full Computer name (FQDN) = el5.wtuto.com

Note: Make sure DNS Server is properly configured.

Install Required package:

yum   install  -y  *postfix*   *dovecot*   *squirrelmail* 

# vim  /etc/postfix/main.cf
inet_interface   =   all
# inet_interface   =  localhost
mydestination =  $myhostname,  localhost.$mydomain, localhost,  $mydomain
home_mailbox =  Maildir/

service  postfix  start

vim  /etc/dovecot.conf
protocols  =  imap  imaps  pop3  pop3s
mail_location  = maildir:~/Maildir
pop3_uidl_format  =  %08Xu%08Xv
imap_client_workarounds = outlook-idle  delay-newmail  netscape-eoh
pop3_client_workarounds = outlook-no-nuls    oe-ns-eoh

service   dovecot   start

vim  /etc/squirrelmail/config.php
$domain                       = ‘wtuto.com’;
$imapServerAddress    = ‘192.168.1.11’;
$smtpServerAddress    = ‘192.168.1.11’;
$default_folder_prefix   = ‘Maildir/’;

service httpd  restart