Sender Policy Framework, or SPF, is a validation system, that is used to prevent the so-called e-mail spoofing where an e-mail message can be sent from one e-mail, but to appear as being sent from another, generally with the objective to scam the recipient someway. When SPF protection is activated for a domain, a special record is created for it in the Domain Name System and all of the DNS servers worldwide get it. The record includes all of the mail servers that are allowed to send authentic messages from an e-mail address under the domain. When a message is sent, the first DNS server it goes to checks the SPF record and when its sending server is permitted, the message is sent to the target receiver. If, however, the sending server doesn't exist in the SPF record for the particular domain, the e-mail message won't be sent and it'll be removed. If you use this service, it will prevent third parties from sending spam emails that appear to have been sent by you.