SPF, which abbreviates Sender Policy Framework, is an authentication system that aims at stopping the so-called e-mail faking. In simple terms, this means sending some message from one email address and making it seem to be sent from another one with the idea to scam in some manner the individual receiving it. In the event that the SPF protection is active for a domain, a record which has 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 direct the web traffic across the globe, therefore they all will recognize whether an e-mail message comes from a legitimate server or not. The verification is performed at the first server where the e-mail goes through and in the first case the message is forwarded, while in the second one it's discarded and it never reaches the intended recipient. Using SPF records for your domain names will prevent all unwanted people from using your email addresses for malicious purposes.