SPF, which abbreviates Sender Policy Framework, is an authentication system that aims at blocking the so-called email forging. In simple terms, it indicates sending some message from one e-mail and making it appear to be sent from a different one with the objective to scam in some way the individual getting it. When the SPF protection is running for a domain name, a record which includes all mail servers authorized to email messages with addresses under the domain is made. The record is kept on all of the DNS servers that route the Internet traffic around the world, so that they all can identify if an e-mail message comes from a legitimate server or not. The check is performed at the very first server where the e-mail goes through and in the first case the message is forwarded, while in the second one it's discarded and it never reaches the supposed recipient. Using SPF records for your domains will stop any unauthorized people from making use of your email addresses for harmful purposes.