SPF, which abbreviates Sender Policy Framework, is a certification system that aims at protecting against the so-called email counterfeiting. Essentially, this means sending a message from one e-mail and making it seem to be sent from another one with the objective to scam in some manner the person opening it. When the SPF protection is running for a domain name, a record which contains all of the mail servers authorized to send messages with addresses part of the domain is created. The record is saved on all DNS servers that direct the Internet traffic globally, thus they all can identify whether an e-mail message originates from a legitimate server or not. The check is performed at the first server where the email goes through and in the first case the e-mail message is forwarded, but in the second one it is removed and it never gets to the supposed recipient. Using SPF records for your domains will stop all unwanted people from using your email addresses for malicious purposes.