SPF, which is an abbreviation for Sender Policy Framework, is a verification system that aims at blocking the so-called email faking. Essentially, this indicates sending a message from one e-mail address and making it seem to be sent from another with the objective to scam in some manner the person receiving it. In the event that the SPF protection is running for a domain, a record which includes all of the mail servers authorized to send messages with addresses under the domain is generated. The record is saved on all of the DNS servers that route the web traffic internationally, thus they all can recognize whether an email message comes from a valid server or not. The verification is done at the first server where the e-mail goes through and in the first case the e-mail message is forwarded, and in the second one it's discarded and it never gets to the intended recipient. Applying SPF records for your domains will stop all unauthorized people from using your email addresses for malicious purposes.