Sender Policy Framework, or SPF, is a verification system, which is employed to stop the so-called email spoofing where an e-mail message can be sent from one address, but to appear as being sent from a different one, often with the idea to scam the recipient in some way. If SPF protection is activated for a domain name, a special record is made for it in the Domain Name System and all the DNS servers around the world get it. The record contains all e-mail servers that are permitted to send valid messages from an e-mail address part of the domain. When an email is sent, the very first DNS server it goes to checks the SPF record and when the sending server is permitted, the message is forwarded to the targeted destination. In case, however, the sending server is not contained in the SPF record for the given domain, the message won't be submitted and it'll be discarded. If you use this service, it'll stop third parties from sending spam messages which seem to have been sent from you.