SPF, which abbreviates Sender Policy Framework, is an authentication system that is aimed at stopping the so-called email counterfeiting. Basically, this means sending some message from one email and making it seem to be sent from another with the purpose to scam in some manner the person receiving it. In case the SPF protection is activated for a domain name, a record that includes all the mail servers authorized to email messages with addresses part of the domain is generated. The record is stored on all the DNS servers that route the web traffic around the world, thus they all can identify if an e-mail message comes 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 email message is forwarded, while in the second one it is removed and it never gets to its intended recipient. Using SPF records for your domains will stop all unwanted people from making use of your email addresses for malicious purposes.