SPF, which abbreviates Sender Policy Framework, is a validation system that is aimed at preventing the so-called e-mail faking. Essentially, it indicates sending some message from one email address and making it appear to be sent from another with the objective to scam in some manner the individual getting it. When the SPF protection is active for a domain, a record that contains all 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 across the globe, thus they all can recognize whether an email message comes from a valid server or not. The check is done at the first server where the email goes through and in the first case the message is forwarded, but in the second it's removed and it never reaches the supposed recipient. Applying SPF records for your domain names will prevent any unwanted people from making use of your emails for malicious purposes.