SPF, which is an abbreviation for Sender Policy Framework, is a certification system that aims at protecting against the so-called e-mail forging. Basically, this indicates sending a message from one email address and making it appear to be sent from another one with the purpose to scam in some manner the individual getting it. In case the SPF protection is running for a domain name, a record which contains all mail servers authorized to email messages with addresses within the domain is created. The record is placed on all of the DNS servers that route the web traffic globally, thus they all can identify whether an email message comes from an authentic server or not. The verification is done at the very first server where the email goes through and in the first case the email message is forwarded, while in the second it's removed and it never reaches the intended recipient. Using SPF records for your domain names will prevent any unauthorized people from making use of your email addresses for malicious purposes.