Sender Policy Framework, or SPF, is a verification system, which is intended to prevent the so-called email spoofing where an e-mail message is sent from one e-mail, but to look as being sent from a different, usually with the idea to fraud the recipient someway. When SPF protection is activated for a domain name, a unique record is generated for it in the Domain Name System and all the DNS servers worldwide get it. The record includes all email servers that are permitted to send legitimate messages from an e-mail address within the domain. When an email is sent, the first DNS server it goes to verifies the SPF record and in case its sending server is authorized, the message is sent to the targeted destination. If, however, the sending server is not present in the SPF record for the specified domain, the email message won't be forwarded and it'll be discarded. Whenever you use this service, it'll prevent third parties from sending spam e-mails which look as if they have been sent from you.