SPF, which is an abbreviation for Sender Policy Framework, is a verification system that is aimed at protecting against the so-called e-mail faking. Basically, this indicates sending some message from one e-mail address and making it appear to be sent from another one with the idea to scam in some way the person getting it. When the SPF protection is active for a domain name, a record that includes all of the mail servers authorized to email messages with addresses under the domain is made. The record is placed on all of the DNS servers that route the web traffic around the world, so they all will recognize whether an email message originates from a valid server or not. The check is done at the first server where the e-mail goes through and in the first case the email message is forwarded, while in the second one it's discarded and it never reaches its intended recipient. Applying SPF records for your domains will stop any unwanted people from using your emails for harmful purposes.