SPF, which abbreviates Sender Policy Framework, is a certification system that aims at protecting against the so-called e-mail counterfeiting. Basically, this means sending a message from one e-mail address and making it seem to be sent from a different one with the objective to scam in some manner the person receiving it. In the event that the SPF protection is running for a domain name, a record which includes all of the mail servers authorized to send messages with addresses part of the domain is made. The record is stored on all of the DNS servers that route the Internet traffic worldwide, so that they all will recognize if an email message comes from an authentic server or not. The check is done at the very first server where the e-mail goes through and in the first case the email message is forwarded, and in the second one it's discarded and it never gets to the intended recipient. Applying SPF records for your domains will stop all unauthorized people from using your emails for harmful objectives.