SPF, which abbreviates Sender Policy Framework, is a validation system that is aimed at preventing the so-called e-mail forging. In simple terms, it indicates sending some message from one email and making it appear to be sent from a different one with the purpose to scam in some way the person opening it. In the event that the SPF protection is running for a domain, a record which contains all the mail servers authorized to email messages with addresses in the domain is made. The record is placed on all DNS servers that route the Internet traffic around the world, so they all will identify if an e-mail message comes from a trustworthy server or not. The check is performed at the very first server where the email goes through and in the first case the message is forwarded, but in the second one it's discarded and it never reaches the intended recipient. Using SPF records for your domain names will prevent any unauthorized people from using your emails for harmful objectives.