SPF, which abbreviates Sender Policy Framework, is an authentication system that is aimed at preventing the so-called email counterfeiting. Essentially, this means sending a message from one email address and making it appear to be sent from a different one with the objective to scam in some manner the person getting it. In case the SPF protection is active for a domain, a record that contains all the mail servers authorized to send messages with addresses under the domain is generated. The record is saved on all the DNS servers that direct the web traffic worldwide, so that they all will identify whether an e-mail message originates from an authentic server or not. The verification is performed at the very first server where the email goes through and in the first case the e-mail message is forwarded, and in the second one it's removed and it never gets to the supposed recipient. Using SPF records for your domain names will prevent any unauthorized people from using your email addresses for malicious objectives.