SPF, which is an abbreviation for Sender Policy Framework, is a validation system that aims at stopping the so-called email forging. Basically, this means sending some message from one email address and making it appear to be sent from a different one with the objective to scam in some way the individual receiving it. In case the SPF protection is running for a domain, a record which has all of the mail servers authorized to email messages with addresses under the domain is created. The record is placed on all of the DNS servers that direct the Internet traffic internationally, so they all can recognize whether an e-mail message originates from an authentic server or not. The check is performed at the very first server where the email goes through and in the first case the email message is forwarded, but in the second one it's discarded and it never gets to the supposed recipient. Applying SPF records for your domains will stop all unwanted people from using your emails for harmful objectives.