Configure A System To Forward All Email To A Central Mail Server

General Information

Configuring postfix.

On the exam, you will be provided all the information of an already configured central SMTP server.

You will be expected to configure a null client only. (server that forwards mail but does not receive any)

Lab Setup

The following virtual machines will be used:

  • ( → Configure SMTP null client (on the exam)
  • ( → Configure central mail server for client testing (NOT on the exam)

Null Client Setup

THIS IS ON THE EXAM: A mail null client forwards local email. It does not receive any mail from network sources.

From server1.

Install required package

yum install postfix

Enable and start the postfix service

systemctl enable postfix
systemctl start postfix

Configure postfix parameters

  • Option A: Use postconf (faster if you have an idea what parameters you need to configure)
    # check a setting
    postconf relayhost
    # check all settings
    postconf | grep <keyword>
    # configure - important
    postconf -e 'relayhost = []'
    # configure - probably already defaults (or set to localhost)
    postconf -e 'inet_interfaces = loopback-only'
    postconf -e 'mydestination ='
    postconf -e 'mydomain ='
  • Option B: Edit the main configuration
    vim /etc/postfix/
    # Relayhost to forward mail to
    relayhost = []
    # Forward from loopback interfaces
    inet_interfaces = loopback-only
    # Configure destination as blank, because we aren't delivering mail locally (only forwarding outgoing)
    mydestination = 
    # Configure domain
    mydomain =

Check postfix syntax

postfix check

Restart the service

systemctl restart postfix

Postfix Central Mail Server

THIS IS NOT ON THE EXAM: This section setups up a mail server to receive mail in order to test the null client configured previously.

From server2

Install postfix

yum install postfix

Enable and start postfix

systemctl enable postfix
systemctl start postfix

Open the firewall to receive SMTP

firewall-cmd --permanent --add-service=smtp
firewall-cmd --reload

Edit the main configuration file

#-- vim directly
vim /etc/postfix/
# listening interfaces
inet_interfaces = all
# Accept mail for these domains
mydestination =,, server2
#-- postconf method
# check settings
postconf | grep inet_
postconf | grep mydestination
# configure
postconf -e 'inet_interfaces = all'
postconf -e 'mydestination =,, server2'

Check postfix syntax

postfix check

Restart the service

systemctl restart postfix

Null Client Testing

From server1

Install a mail client (if not already installed)

yum install mailx

Send a test message

echo "Did it work?" | mail -s "This is a postfix forward test"

Watch the mail log file for status messages

tail -f /var/log/maillog

From server2

On the postfix relayhost, check root's mail



On the sending client, to view the mail queue

postqueue -p

Flush the sending mail queue after fixing a problem to get rid of stuck messages

postqueue -f

If a message is stuck and won't flush, it can be removed

postsuper -d <queue_id>
  • Instead of a single queue_id, you can specify the keyword 'ALL'

On the receiving server, check the maillog for reasons a message did not deliver

tail /var/log/maillog

  • linux_wiki/configure_a_system_to_forward_all_email_to_a_central_mail_server.txt
  • Last modified: 2019/05/25 23:50
  • (external edit)