Sender Policy Framework, or SPF, is a validation system, that is designed to stop the so-called email spoofing where an e-mail message is sent from one email address, but to look as being sent from another one, often with the objective to fraud the recipient for some reason. When SPF protection is activated for a domain, a special record is generated for it in the Domain Name System and all DNS servers globally receive it. The record includes all the mail servers that are allowed to send legitimate messages from an e-mail address part of the domain. When a message is sent, the very first DNS server it goes to verifies the SPF record and when its sending server is permitted, the message is sent to the target receiver. If, however, the sending server is not included in the SPF record for the given domain, the message won't be forwarded and it'll be removed. Whenever you use this service, it will prevent third parties from sending spam that appear to have been sent by you.