SPF, which is an abbreviation for Sender Policy Framework, is an authentication system that is aimed at protecting against the so-called email counterfeiting. In simple terms, this means sending some message from one e-mail and making it seem to be sent from another one with the purpose to scam in some way the person getting it. When the SPF protection is active for a domain, a record which contains all mail servers authorized to send messages with addresses in the domain is generated. The record is kept on all DNS servers that route the web traffic globally, so they all will identify whether an e-mail message originates from a trustworthy 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, but in the second one it's removed and it never gets to the intended recipient. Applying SPF records for your domain names will stop any unwanted people from using your emails for harmful purposes.