SPF, which is an abbreviation for Sender Policy Framework, is a verification system that is aimed at stopping the so-called email forging. Essentially, it indicates sending some message from one email and making it seem to be sent from another one with the purpose to scam in some way the person receiving it. In the event that the SPF protection is running for a domain, a record which has all the mail servers authorized to send messages with addresses in the domain is made. The record is kept on all DNS servers that direct the web traffic internationally, therefore they all can recognize whether an e-mail message originates from a trustworthy server or not. The check is done at the very first server where the e-mail goes through and in the first case the email message is forwarded, and in the second one it's removed and it never reaches the supposed recipient. Applying SPF records for your domains will prevent all unwanted people from using your email addresses for malicious purposes.