SPF, which is an abbreviation for Sender Policy Framework, is an authentication system that is aimed at protecting against the so-called e-mail forging. Basically, it indicates sending a message from one email address and making it look like it’s sent from a different one with the objective to scam in some manner the individual opening it. In case the SPF protection is active for a domain, a record which includes all the mail servers authorized to email messages with addresses part of the domain is generated. The record is placed on all DNS servers that route the web traffic across the globe, therefore they all can recognize if an email message comes from an authentic server or not. The check is conducted at the first server where the e-mail goes through and in the first case the email message is forwarded, but in the second one it's removed and it never reaches the intended recipient. Using SPF records for your domain names will stop any unwanted people from making use of your email addresses for harmful purposes.