SPF, which is an abbreviation for Sender Policy Framework, is a certification system that is aimed at protecting against the so-called email counterfeiting. Basically, it indicates sending a message from one email and making it appear to be sent from another one with the idea to scam in some manner the person receiving it. In the event that the SPF protection is activated for a domain, a record that includes all the mail servers authorized to email messages with addresses part of the domain is created. The record is placed on all of the DNS servers that direct the Internet traffic globally, so they all will recognize whether an e-mail message originates from a trustworthy server or not. The check is performed at the very first server where the email goes through and in the first case the email message is forwarded, whereas in the second it's discarded and it never gets to its intended recipient. Applying SPF records for your domains will prevent any unwanted people from using your emails for harmful purposes.