SPF, which abbreviates Sender Policy Framework, is a certification system that aims at protecting against the so-called e-mail counterfeiting. Essentially, this means sending some message from one email and making it look like it’s sent from another with the objective to scam in some way the person getting it. If the SPF protection is activated for a domain name, a record that includes all mail servers authorized to email messages with addresses under the domain is made. The record is placed on all of the DNS servers that route the web traffic worldwide, so that they all can recognize if an e-mail message originates from a legitimate server or not. The verification is done at the first server where the email goes through and in the first case the message is forwarded, but in the second one it's discarded and it never reaches the supposed recipient. Using SPF records for your domain names will prevent any unauthorized people from using your emails for harmful purposes.