The mail routing section allows you to select what domains you would like aliased. If you have a user at the guardiandigital.com domain, and want every user to be able to receive mail to linuxsecurity.com as well, this menu provides that ability. Refer to Figure 4.4.5.
Enter in the domain you want the mail aliased as. We used linuxsecurity.com to create an existing mail route in the above image. We then enter in the Relay mail to... field the actual domain the mail should go to, guardiandigital.com in this example.