SPF, which abbreviates Sender Policy Framework, is a certification system that is aimed at protecting against the so-called email forging. Basically, this means sending a message from one e-mail address and making it appear to be sent from a different one with the purpose to scam in some way the person getting it. In case the SPF protection is running for a domain, a record that contains all the mail servers authorized to send messages with addresses within the domain is made. The record is placed on all the DNS servers that route the Internet traffic all over the world, thus they all will identify whether an email message originates from an authentic server or not. The check is done at the first server where the email goes through and in the first case the email message is forwarded, but in the second it is discarded and it never reaches the supposed recipient. Employing SPF records for your domain names will prevent all unauthorized people from making use of your emails for malicious purposes.