Sender Policy Framework, or SPF, is a verification system, which is employed to stop the so-called e-mail spoofing where an email message is sent from one e-mail address, but to appear as being sent from another one, generally with the objective to scam the recipient in some way. If SPF protection is enabled for a domain, a unique record is created for it in the Domain Name System and all the DNS servers worldwide have it. The record includes all e-mail servers that are allowed to send legitimate messages from an e-mail address under the domain. When an e-mail is sent, the first DNS server it encounters verifies the SPF record and in case the sending server is permitted, the message is forwarded to the target receiver. When, however, the sending server is not contained in the SPF record for the given domain, the email message won't be sent and it'll be discarded. Whenever you use this service, it will prevent third parties from sending spam messages that seem to have been sent by you.