SPF, which is an abbreviation for Sender Policy Framework, is a certification system that is aimed at preventing the so-called email counterfeiting. Basically, it indicates sending a message from one e-mail and making it seem to be sent from another one with the idea to scam in some way the person opening it. If the SPF protection is active for a domain, a record which has all mail servers authorized to email messages with addresses under the domain is made. The record is kept on all the DNS servers that route the Internet traffic worldwide, so they all can recognize whether an email message originates from a legitimate 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, but in the second one it's removed and it never reaches its intended recipient. Employing SPF records for your domain names will stop all unauthorized people from making use of your emails for harmful purposes.