SPF, which abbreviates Sender Policy Framework, is a validation system that aims at preventing the so-called e-mail counterfeiting. Essentially, it indicates sending some message from one email and making it appear to be sent from another with the purpose to scam in some manner the individual receiving it. When the SPF protection is running for a domain, a record which contains all of the mail servers authorized to send messages with addresses under the domain is made. The record is kept on all of the DNS servers that direct the web traffic all over the world, therefore they all can recognize if an e-mail 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 message is forwarded, but in the second it's removed and it never gets to the intended recipient. Applying SPF records for your domain names will prevent all unauthorized people from making use of your emails for harmful purposes.