Sender Policy Framework, or SPF, is a verification system, that is intended to stop the so-called e-mail spoofing where an email message can be sent from one e-mail address, but to appear as being sent from a different one, typically with the idea to scam the recipient for some reason. When SPF protection is enabled for a domain name, a specific record is generated for it in the Domain Name System and all the DNS servers worldwide have it. The record contains all of the e-mail servers that are allowed to send legitimate messages from an email address part of the domain. When a message is sent, the very first DNS server it encounters checks the SPF record and if the sending server is authorized, the message is forwarded to the target receiver. When, however, the sending server doesn't exist in the SPF record for the specified domain, the email message will not be forwarded and it'll be removed. In case you use this service, it will prevent third parties from sending spam that look as if they have been sent by you.