SPF, which abbreviates Sender Policy Framework, is an authentication system that aims at blocking the so-called email faking. Essentially, it indicates sending a message from one email and making it appear to be sent from a different one with the objective to scam in some way the person getting it. In case the SPF protection is activated for a domain, a record which includes all the mail servers authorized to email messages with addresses within the domain is made. The record is kept on all of the DNS servers that route the web traffic all over the world, therefore they all can recognize whether an e-mail message originates from an authentic server or not. The check is conducted at the very 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's removed and it never gets to the supposed recipient. Using SPF records for your domains will prevent all unwanted people from making use of your email addresses for harmful purposes.