SPF, which abbreviates Sender Policy Framework, is an authentication system that aims at blocking the so-called email forging. In simple terms, this indicates sending a message from one email and making it appear to be sent from a different one with the idea to scam in some way the individual opening it. In the event that the SPF protection is active for a domain, a record which has all the mail servers authorized to email messages with addresses within the domain is created. The record is placed on all DNS servers that route the Internet traffic across the globe, so they all will recognize if an e-mail message comes from a trustworthy server or not. The verification is conducted at the first server where the email goes through and in the first case the email message is forwarded, but in the second it's discarded and it never gets to its intended recipient. Using SPF records for your domain names will prevent all unwanted people from making use of your emails for harmful purposes.