SPF, which is an abbreviation for Sender Policy Framework, is a certification system that aims at protecting against the so-called e-mail forging. Basically, it indicates sending a message from one e-mail and making it appear to be sent from a different one with the objective to scam in some manner the person getting it. When the SPF protection is active for a domain name, a record which has all mail servers authorized to send messages with addresses part of the domain is made. The record is placed on all DNS servers that route the Internet traffic around the world, so they all will identify whether an email message comes from a valid server or not. The check is conducted at the first server where the email goes through and in the first case the e-mail message is forwarded, but in the second one it's discarded and it never gets to the intended recipient. Applying SPF records for your domain names will stop all unauthorized people from using your emails for malicious objectives.