SPF, which abbreviates Sender Policy Framework, is a certification system that is aimed at stopping the so-called e-mail faking. Basically, this indicates sending a message from one e-mail address and making it seem to be sent from another one with the idea to scam in some way the person opening it. In case the SPF protection is activated for a domain, a record which contains all mail servers authorized to send messages with addresses within the domain is generated. The record is kept on all of the DNS servers that direct the web traffic all over the world, therefore they all can recognize whether an e-mail message originates from a trustworthy server or not. The check is done at the first server where the email goes through and in the first case the e-mail message is forwarded, and in the second it's removed and it never reaches the supposed recipient. Applying SPF records for your domain names will prevent all unauthorized people from making use of your email addresses for malicious purposes.