SPF, which is an abbreviation for Sender Policy Framework, is a validation system that is aimed at stopping the so-called e-mail forging. Essentially, this indicates sending some message from one email address and making it seem to be sent from another one with the purpose to scam in some way the individual receiving it. When the SPF protection is running for a domain, a record which contains all mail servers authorized to send messages with addresses in the domain is made. The record is stored on all DNS servers that route the Internet traffic around the world, therefore they all will identify if an email message originates from a trustworthy server or not. The verification is performed at the very first server where the e-mail goes through and in the first case the email message is forwarded, and in the second one it's removed and it never gets to the intended recipient. Applying SPF records for your domain names will stop any unwanted people from using your email addresses for harmful purposes.