Sender Policy Framework, or SPF, is a verification system, that is designed to stop the so-called email spoofing where an email message is sent from one e-mail, but to appear as being sent from another one, often with the objective to scam the recipient someway. When SPF protection is activated for a domain, a specific record is created for it in the Domain Name System and all DNS servers globally get it. The record contains all e-mail servers that are permitted to send valid messages from an address under the domain. When an email is sent, the first DNS server it goes to verifies the SPF record and when its sending server is authorized, the message is sent to the target destination. In case, however, the sending server is not present in the SPF record for the given domain, the message won't be sent and it will be removed. If you use this service, it will stop third parties from sending spam messages which look as if they have been sent from you.