About SPF: SPF (Sender Policy Framework)  is an open standard specifying a technical method that was created in order to stop and eliminate the forged or spoofed sender email addresses in the mail envelope SMTP MAIL FROM or Return-Path that commonly used in spam message.

SPF allows the owner of an Internet domain to use special format of DNS records (“SPF”) to specify which machines are authorized to transmit e-mail for that domain. For example, the owner of the example.net domain can designate which machines are authorized to send e-mail whose sender e-mail address ends with “@example.net”.

A typical example TXT record for SPF looks like this:

example.net.  TXT  “v=spf1 mx a:pluto.example.net include:aspmx.googlemail.com -all”
The parts of the SPF record mean the following:

v=spf1    SPF version 1
mx    the incoming mail servers (MXes) of the domain are authorized to also send mail for example.net
a: pluto.example.net    the machine pluto.example.net is authorized, too
include:aspmx.googlemail.com    everything considered legitimate by gmail.com is legitimate for example.net, too
-all    all other machines are not authorized

To check if your SPF record is correct, there are various SPF checker, tester or validator available like http://www.kitterman.com/spf/validate.html