SPF, which is an abbreviation for Sender Policy Framework, is a validation system that is aimed at preventing the so-called e-mail counterfeiting. In simple terms, this indicates sending a message from one e-mail address and making it look like it’s sent from another one 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 mail servers authorized to email messages with addresses within the domain is made. The record is placed on all the DNS servers that route the web traffic all over the world, so they all can identify if an email message comes from a legitimate server or not. The check is performed at the very first server where the e-mail goes through and in the first case the message is forwarded, while in the second one it's discarded and it never gets to its intended recipient. Using SPF records for your domains will stop all unwanted people from making use of your emails for harmful purposes.