SPF, which abbreviates Sender Policy Framework, is a verification system that is aimed at protecting against the so-called e-mail forging. In simple terms, it indicates sending a message from one email and making it seem to be sent from another one with the objective to scam in some way the individual getting it. In the event that the SPF protection is active for a domain name, a record which includes all of the mail servers authorized to email messages with addresses part of the domain is made. The record is kept on all DNS servers that route the Internet traffic across the globe, thus they all will recognize whether an e-mail message originates from a valid server or not. The verification is performed at the first server where the e-mail goes through and in the first case the email message is forwarded, and in the second it's discarded and it never reaches the supposed recipient. Applying SPF records for your domains will stop all unwanted people from making use of your email addresses for harmful purposes.