Sender Policy Framework, or SPF, is a verification system, that is employed to prevent the so-called email spoofing where an e-mail message is sent from one e-mail address, but to look as being sent from another, usually with the objective to fraud the recipient in some way. If SPF protection is activated for a domain, a special record is generated for it in the Domain Name System and all of the DNS servers around the world have it. The record contains all the mail servers that are permitted to send valid messages from an email address part of the domain. When a message is sent, the very first DNS server it goes to checks the SPF record and if its sending server is authorized, the message is sent to the target receiver. In case, however, the sending server is not contained in the SPF record for the specified domain, the message won't be sent and it'll be removed. In the event that you employ this service, it'll stop third parties from sending spam emails which seem to have been sent by you.