Sender Policy Framework, or SPF, is a validation system, which is employed to prevent the so-called email spoofing where an e-mail message is sent from one email, but to look as being sent from another, often with the idea to scam the recipient someway. When SPF protection is activated for a domain, a special record is made for it in the Domain Name System and all of the DNS servers worldwide receive it. The record includes all of the e-mail servers that are permitted to send legitimate messages from an e-mail address part of the domain. When a message is sent, the first DNS server it goes to checks the SPF record and if its sending server is authorized, the message is sent to the target receiver. If, however, the sending server is not contained in the SPF record for the particular domain, the email message won't be submitted and it'll be removed. If you use this service, it'll prevent third parties from sending spam e-mails which appear to have been sent by you.