Configure Postfix if Necessary

/etc/postfix/main.cf

$ sudo postconf -e 'smtpd_client_restrictions = permit_mynetworks,reject'

• If any system in the subnet containing the MTA may be trusted to relay messages, add or correct the following line:

  mynetworks_style = subnet

  This is also the default setting, and is in effect if all my_networks_style directives are commented.
• If only the MTA host itself is trusted to relay messages, add or correct the following line:

  mynetworks_style = host

• If the set of systems which can relay is more complicated, manually specify an entry for each netblock or IP address which is trusted to relay by setting the mynetworks variable directly:

  mynetworks = 10.0.0.0/16, 192.168.1.0/24, 127.0.0.1