Sender Policy Framework, or SPF, is a validation system, that is designed to prevent the so-called email spoofing where an email message is sent from one email, but to appear as being sent from another, generally with the objective to scam the recipient someway. When SPF protection is enabled for a domain name, a specific record is made for it in the Domain Name System and all the DNS servers around the world get it. The record features all mail servers that are allowed to send legitimate messages from an email address within the domain. When a message is sent, the very first DNS server it goes to verifies the SPF record and in case its sending server is permitted, the message is forwarded to the target destination. In case, however, the sending server is not included in the SPF record for the specified domain, the e-mail message will not be forwarded and it'll be removed. In case you employ this service, it'll prevent third parties from sending spam e-mails which look as if they have been sent by you.