The Sendmail program is a very widely used Mail Transport Agent (MTA). MTAs send mail from one machine to another. Sendmail is not a client program, which you use to read your email. Sendmail is a behind-the-scenes program which actually moves your email over networks or the Internet to where you want it to go..  This guide show you have to install and configure sendmail with virtual user and domains -  Installation  - Check if sendmail is installed or not: # rpm –qa | grep sendmail If it is not installed, then install the sendmail package using following command: # yum install sendmail sendmail-cf By default sendmail listen on local interface i.e 127.0.0.1 To configure the sendmail to listen to all interface at the host, just comment the following line in “sendmail.mc”: # vi /etc/mail/sendmail.mc From: DAEMON_OPTIONS(`Port=smtp,Addr=127.0.0.1, Name=MTA')dnl ...