Configure a Null Client Mail Server on RHEL / CentOS 7

A null client is a client machine that runs a local mail server which forwards all emails to an outbound mail relay server for delivery. A null client does not accept local delivery for any messages, it can only send them to the outbound mail relay server. Users may run mail clients on the null client to read and send emails.

In Red Hat Enterprise Linux (RHEL) 7, default mail server is postfix (a powerful but easy-to-configure mail server). In this article, we will configure a null client using postfix mail server.

 

System Specification:

There is no special requirement for null client. Therefore, we are using a virtual machine with following specification.

• Operating System - Red Hat Enterprise Linux (RHEL) 7
• Hostname - server1.example.com
• IP Address - 192.168.116.12/24
• Email Relay Host - 192.168.116.5

 

Installing Postfix on RHEL 7:

Connect to server1.example.com using ssh, and install postfix using yum command.

[root@server1 ~]# yum install -y postfix
Loaded plugins: langpacks, product-id, subscription-manager
This system is not registered to Red Hat Subscription Management. You can use subscription-manager to register.
Resolving Dependencies
--> Running transaction check
---> Package postfix.x86_64 2:2.10.1-6.el7 will be installed
--> Finished Dependency Resolution

Dependencies Resolved

==========================================================================================================================
 Package                    Arch                      Version                           Repository                   Size
==========================================================================================================================
Installing:
 postfix                    x86_64                    2:2.10.1-6.el7                    localyum                    2.4 M

Transaction Summary
==========================================================================================================================
Install  1 Package

Total download size: 2.4 M
Installed size: 12 M
Downloading packages:
Running transaction check
Running transaction test
Transaction test succeeded
Running transaction
  Installing : 2:postfix-2.10.1-6.el7.x86_64                                                                          1/1
  Verifying  : 2:postfix-2.10.1-6.el7.x86_64                                                                          1/1

Installed:
  postfix.x86_64 2:2.10.1-6.el7

Complete!

Configure postfix using postconf command.

[root@server1 ~]# postconf -e 'relayhost=[192.168.116.5]'
[root@server1 ~]# postconf -e 'inet_interfaces=loopback-only'
[root@server1 ~]# postconf -e 'myorigin=server1.example.com'
[root@server1 ~]# postconf -e 'mydestination='
[root@server1 ~]# postconf -e 'local_transport=error: local delivery disabled'
[root@server1 ~]# postconf -e 'mynetworks=127.0.0.0/8,[::1]/128'
[root@server1 ~]# systemctl start postfix.service ; systemctl enable postfix.service

Our null client mail server on RHEL 7 has been configured successfully.