SPF, which abbreviates Sender Policy Framework, is an authentication system that aims at stopping the so-called email forging. In simple terms, this means sending a 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 receiving it. In the event that the SPF protection is running for a domain, a record that contains all of the mail servers authorized to email messages with addresses within the domain is created. The record is saved on all the DNS servers that route the web traffic across the globe, so they all can recognize whether an email message comes from an authentic server or not. The check is performed at the first server where the e-mail goes through and in the first case the message is forwarded, and in the second one it's discarded and it never reaches its intended recipient. Applying SPF records for your domains will stop any unwanted people from making use of your email addresses for harmful purposes.