SPF, which abbreviates Sender Policy Framework, is a verification system that is aimed at protecting against the so-called e-mail faking. In simple terms, it indicates sending some message from one email address and making it seem to be sent from a different one with the objective to scam in some way the person opening it. If the SPF protection is running for a domain, a record which has all of the mail servers authorized to send messages with addresses under the domain is made. The record is kept on all the DNS servers that direct the web traffic globally, so that they all can identify if an email message originates from an authentic server or not. The verification is conducted 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 discarded and it never reaches its intended recipient. Using SPF records for your domain names will prevent any unwanted people from using your emails for malicious purposes.