SPF, which is an abbreviation for Sender Policy Framework, is a verification system that is aimed at preventing the so-called e-mail faking. In simple terms, this means sending some message from one email and making it seem to be sent from a different one with the purpose to scam in some manner the individual getting it. If the SPF protection is running for a domain name, a record which has all the mail servers authorized to send messages with addresses under the domain is created. The record is placed on all of the DNS servers that direct the Internet traffic globally, so they all will identify whether an e-mail message originates from a legitimate server or not. The verification is conducted at the first server where the email goes through and in the first case the message is forwarded, while in the second one it's discarded and it never gets to the intended recipient. Applying SPF records for your domains will stop any unwanted people from using your emails for harmful purposes.