To set up support for Sender Policy Framework on your Linux-based server:
We recommend that you add a rule for checking messages against the open database of trusted senders, for example, 'include:spf.trusted-forwarder.org'. For more information on SPF rules, visit http://tools.ietf.org/html/rfc4408.
For example: v=spf1 +a/24 +mx/24 +ptr ?all
If no value is specified, the default text will be used as a notification.