SPF, which is an abbreviation for Sender Policy Framework, is a certification system that is aimed at blocking the so-called e-mail faking. Basically, this indicates sending some message from one e-mail and making it seem to be sent from a different one with the purpose to scam in some way the person getting it. If the SPF protection is running for a domain, a record which includes all the mail servers authorized to send messages with addresses in the domain is created. The record is placed on all the DNS servers that direct the web traffic internationally, therefore they all will identify if an e-mail message comes from a trustworthy server or not. The check is performed at the first server where the e-mail goes through and in the first case the e-mail message is forwarded, while in the second it's discarded and it never gets to the supposed recipient. Applying SPF records for your domains will stop any unauthorized people from making use of your email addresses for harmful purposes.