Sender Policy Framework, or SPF, is a verification system, that is used to stop the so-called email spoofing where an e-mail message is sent from one email address, but to seem as being sent from another one, generally with the objective to fraud the recipient in some way. When SPF protection is activated for a domain, a unique record is made for it in the Domain Name System and all of the DNS servers worldwide get it. The record includes all the e-mail servers that are allowed to send legitimate messages from an email address part of the domain. When an e-mail is sent, the first DNS server it goes to checks the SPF record and when its sending server is authorized, the message is forwarded to the target receiver. When, however, the sending server is not included in the SPF record for the given domain, the email message won't be submitted and it will be removed. In case you use this service, it'll prevent third parties from sending spam emails which appear to have been sent by you.