Sender Policy Framework, or SPF, is a validation system, which is designed to stop the so-called email spoofing where an email message can be sent from one email, but to seem as being sent from another, usually with the idea to scam the recipient for some reason. When SPF protection is activated for a domain, a unique record is generated for it in the Domain Name System and all the DNS servers worldwide have it. The record features all e-mail servers that are allowed to send legitimate messages from an address within the domain. When a message is sent, the very first DNS server it goes to verifies the SPF record and in case the sending server is authorized, the message is forwarded to the target destination. If, however, the sending server doesn't exist in the SPF record for the particular domain, the e-mail message won't be forwarded and it'll be removed. Whenever you use this service, it will stop third parties from sending spam messages that seem to have been sent by you.