SPF, which abbreviates Sender Policy Framework, is a certification system that aims at preventing the so-called email forging. Essentially, it indicates sending a message from one e-mail address and making it appear to be sent from another with the purpose to scam in some way the individual getting it. In case the SPF protection is running for a domain, a record which has all of the mail servers authorized to send messages with addresses part of the domain is made. The record is stored on all DNS servers that route the Internet traffic worldwide, so that they all will recognize if an email message originates from a valid server or not. The check is done at the very first server where the email goes through and in the first case the e-mail message is forwarded, while in the second one it is removed and it never reaches the intended recipient. Applying SPF records for your domains will stop any unwanted people from using your email addresses for harmful purposes.