SPF, which abbreviates Sender Policy Framework, is a verification system that aims at stopping the so-called e-mail counterfeiting. Basically, this means sending some message from one email address and making it appear to be sent from a different one with the idea to scam in some way the individual getting it. If the SPF protection is running for a domain, a record which contains all the mail servers authorized to email messages with addresses within the domain is generated. The record is kept on all the DNS servers that direct the Internet traffic worldwide, thus they all can identify if an e-mail message originates from a trustworthy server or not. The check is conducted at the very 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 intended recipient. Applying SPF records for your domains will prevent all unauthorized people from using your emails for harmful purposes.