SPF, which is an abbreviation for Sender Policy Framework, is a verification system that is aimed at preventing the so-called email forging. Basically, this indicates sending a message from one email and making it seem to be sent from another with the idea to scam in some way the individual opening it. If the SPF protection is active for a domain, a record which contains all mail servers authorized to email messages with addresses within the domain is created. The record is saved on all DNS servers that route the web traffic around the world, so they all will recognize if an email message comes from a legitimate server or not. The check is done at the first server where the e-mail goes through and in the first case the e-mail message is forwarded, but in the second one it is removed and it never reaches its intended recipient. Using SPF records for your domains will prevent any unauthorized people from using your email addresses for harmful purposes.