Sender Policy Framework, or SPF, is a verification system, which is designed to prevent the so-called email spoofing where an e-mail message can be sent from one email, but to appear as being sent from a different, usually with the idea to scam the recipient someway. When SPF protection is activated for a domain, a specific record is made for it in the Domain Name System and all of the DNS servers around the world have it. The record features all of the mail servers that are permitted to send valid messages from an email address under the domain. When an email is sent, the first DNS server it encounters verifies the SPF record and when its sending server is permitted, the message is forwarded to the targeted destination. When, however, the sending server is not included in the SPF record for the specified domain, the e-mail message won't be forwarded and it will be discarded. In case you use this service, it'll stop third parties from sending spam that appear to have been sent from you.