Sender Policy Framework, or SPF, is a verification system, that is employed to prevent the so-called email spoofing where an e-mail message can be sent from one email address, but to look as being sent from a different, frequently with the purpose to fraud the recipient for some reason. If SPF protection is enabled for a domain name, a specific record is created for it in the Domain Name System and all the DNS servers globally receive it. The record contains all of the email servers that are permitted to send valid messages from an email address part of the domain. When an email is sent, the first DNS server it goes to verifies the SPF record and if 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 particular domain, the message won't be forwarded and it'll be discarded. If you use this service, it will stop third parties from sending spam that seem to have been sent from you.