SPF, which is an abbreviation for Sender Policy Framework, is a certification system that is aimed at stopping the so-called email counterfeiting. Basically, it indicates sending some message from one e-mail address and making it seem to be sent from another with the purpose to scam in some manner the individual opening it. In case the SPF protection is activated for a domain name, a record which contains all of the mail servers authorized to send messages with addresses part of the domain is made. The record is placed on all DNS servers that direct the Internet traffic internationally, therefore they all can identify whether an e-mail message originates from a legitimate server or not. The verification is conducted at the very first server where the e-mail goes through and in the first case the e-mail message is forwarded, whereas in the second one it's discarded and it never reaches the supposed recipient. Using SPF records for your domains will prevent any unwanted people from using your email addresses for harmful purposes.