SPF, which is an abbreviation for Sender Policy Framework, is an authentication system that aims at blocking the so-called email counterfeiting. In simple terms, it indicates sending a message from one email address and making it seem to be sent from another with the purpose to scam in some way the individual opening it. If the SPF protection is active for a domain, a record which has all mail servers authorized to send messages with addresses within the domain is made. The record is placed on all DNS servers that direct the web traffic around the world, so that they all will recognize if an e-mail message comes from an authentic server or not. The verification is conducted at the first server where the e-mail goes through and in the first case the email message is forwarded, and in the second one it's discarded and it never gets to its intended recipient. Using SPF records for your domains will prevent all unauthorized people from making use of your emails for harmful purposes.