SPF, which is an abbreviation for Sender Policy Framework, is an authentication system that is aimed at blocking the so-called email faking. In simple terms, this means sending some message from one e-mail and making it appear to be sent from another with the purpose to scam in some manner the individual receiving it. In case the SPF protection is active for a domain, a record that includes all mail servers authorized to send messages with addresses under the domain is created. The record is stored on all of the DNS servers that direct the Internet traffic across the globe, so that they all will identify whether an email message comes from a valid server or not. The check is conducted at the first server where the email goes through and in the first case the message is forwarded, while in the second one it's discarded and it never reaches its intended recipient. Applying SPF records for your domains will prevent any unauthorized people from making use of your email addresses for harmful purposes.