SPF, which is an abbreviation for Sender Policy Framework, is a validation system that is aimed at blocking the so-called email counterfeiting. Basically, it indicates sending a message from one e-mail and making it appear to be sent from another one with the purpose to scam in some way the person receiving it. When the SPF protection is active for a domain name, a record which includes all mail servers authorized to send messages with addresses within the domain is created. The record is placed on all the DNS servers that route the web traffic around the world, therefore they all can identify if an email message comes from a valid server or not. The check is performed at the first server where the email goes through and in the first case the email message is forwarded, whereas in the second it's removed and it never reaches the supposed recipient. Applying SPF records for your domain names will prevent any unauthorized people from making use of your email addresses for malicious objectives.