SPF, which is an abbreviation for Sender Policy Framework, is a validation system that is aimed at preventing the so-called email faking. Essentially, this means sending some message from one e-mail and making it seem to be sent from another with the idea to scam in some manner the person getting it. When the SPF protection is active for a domain name, a record which has all mail servers authorized to email messages with addresses within the domain is generated. The record is kept on all of the DNS servers that route the web traffic internationally, thus they all can identify if an e-mail message originates from a valid server or not. The verification is done at the first server where the email goes through and in the first case the message is forwarded, but in the second it's removed and it never reaches the supposed recipient. Using SPF records for your domains will stop any unwanted people from using your emails for malicious purposes.