SPF, which abbreviates Sender Policy Framework, is a validation system that is aimed at blocking the so-called email faking. Essentially, this indicates sending some message from one email address and making it appear to be sent from another one with the purpose to scam in some way the individual receiving it. In case the SPF protection is running for a domain, a record that includes all of the mail servers authorized to send messages with addresses part of the domain is created. The record is kept on all DNS servers that direct the web traffic around the world, so that they all can identify if an e-mail message comes from a trustworthy server or not. The verification is done at the very first server where the e-mail goes through and in the first case the email message is forwarded, while in the second it's discarded and it never gets to the supposed recipient. Applying SPF records for your domain names will stop all unauthorized people from using your emails for harmful purposes.