SPF, which is an abbreviation for Sender Policy Framework, is an authentication system that is aimed at blocking the so-called email counterfeiting. Basically, this means sending some message from one e-mail address and making it look like it’s sent from another with the purpose to scam in some way the person opening it. When the SPF protection is active for a domain, a record which includes all mail servers authorized to email messages with addresses within the domain is created. The record is kept on all DNS servers that route the Internet traffic all over the world, therefore they all can identify whether an e-mail message comes from an authentic server or not. The verification is done at the first server where the email goes through and in the first case the email message is forwarded, and in the second it's removed and it never reaches the supposed recipient. Using SPF records for your domain names will prevent any unwanted people from making use of your email addresses for harmful purposes.