Sender Policy Framework, or SPF, is a validation system, which is employed to stop the so-called e-mail spoofing where an e-mail message is sent from one e-mail address, but to look as being sent from a different one, generally with the objective to scam the recipient for some reason. If SPF protection is activated for a domain, a unique record is generated for it in the Domain Name System and all the DNS servers worldwide receive it. The record includes all the email servers that are allowed to send valid messages from an e-mail address within the domain. When an email is sent, the very first DNS server it goes to checks the SPF record and if the sending server is authorized, the message is sent to the targeted destination. In case, however, the sending server doesn't exist in the SPF record for the given domain, the message will not be sent and it'll be discarded. If you use this solution, it'll prevent third parties from sending spam e-mails that seem to have been sent by you.