SPF, which is an abbreviation for Sender Policy Framework, is a verification system that is aimed at protecting against the so-called email counterfeiting. In simple terms, this means sending a message from one e-mail address and making it seem to be sent from a different one with the idea to scam in some manner the person opening it. In case the SPF protection is active for a domain name, a record which has all the mail servers authorized to send messages with addresses part of the domain is made. The record is kept on all DNS servers that direct the Internet traffic worldwide, thus they all will identify whether an e-mail message originates from a trustworthy server or not. The verification is done at the very first server where the e-mail goes through and in the first case the e-mail message is forwarded, whereas in the second one it is removed and it never gets to the intended recipient. Using SPF records for your domains will stop all unwanted people from making use of your email addresses for malicious purposes.