When you set up Postfix it is a critical service for your organization.  It is important that you set up a way to verify that the mail server is up and running.  Nagios 3 provides an easy set up to allow you to monitor your mail server.  This tutorial  will help you understand how to add mail service checks for your Postfix Mail Server.  If you need help in setting up Nagios 3 please check THIS ARTICLE.

Step #1: Add a Host
An easy way to start setting up hosts is to choose a web server to monitor.
You will need to edit /etc/nagios3/Define your host, give it a host name and an alias, be sure to have the correct IP Address.  Use the check_http command which will monitor your web server on port 80 tcp.  This is a much easier way to monitor a web server using icmp because you have to modify so many firewalls to allow icmp.

define host{
use                             generic-host
host_name                       mail
alias                           mail
address                         192.168.5.12
check_command                   check_smtp
max_check_attempts              10
notification_interval           120
notification_period             24×7
notification_options            d,u,r
contact_groups                  admins
}

Step #2: Add Host to a Service

Edit the /etc/nagios3/conf.d/generic-service_nagios2.cfg.  If you are using the same service you can just add the second host to the host_name line.  This will make it very easy to add a number of hosts to modify.  Note two host names listed.

define service{
use     generic-service
host_name       mail
service_description     SMTP
check_command   check_smtp
}

Step #3: Check Configuration and Restart
You will want to run this command to check your pre-flight check to verify you do not have typos or other errors.

nagios -v /etc/nagios3/nagios.cfg

This should result in no errors and no warnings before you proceed.

Now restart nagios and the web server for nagios.

/etc/init.d/nagios3 restart
/etc/init.d/apache2 restart

Now access the web interface of Nagios at:

http://your_ip_address/nagios3

You should see that the service is being monitored.