SPF, which is an abbreviation for Sender Policy Framework, is an authentication system that aims at stopping the so-called e-mail counterfeiting. In simple terms, this means sending a message from one e-mail and making it appear to be sent from another with the purpose to scam in some manner the individual opening it. If the SPF protection is active for a domain name, a record that contains all of the mail servers authorized to email messages with addresses under the domain is made. The record is stored on all DNS servers that direct the web traffic internationally, thus they all can recognize if an email message originates from a valid server or not. The verification is done at the first server where the e-mail goes through and in the first case the email message is forwarded, but in the second it's removed and it never reaches its intended recipient. Using SPF records for your domains will prevent all unwanted people from making use of your emails for harmful objectives.