SPF, which is short for Sender Policy Framework, is an email safety system, that is used to verify whether an email message was sent by an authorized server. Employing SPF protection for a given domain will prevent the faking of emails made with the domain. In layman's terms: activating this attribute for a domain name makes a particular record in the Domain Name System (DNS) containing the IP addresses of the servers which are permitted to send email messages from mail boxes under the domain. The moment this record propagates worldwide, it will exist on all of the DNS servers that direct the Internet traffic. Every time a new email message is sent, the first DNS server it uses tests if it originates from an official server. If it does, it is forwarded to the destination address, yet if it does not originate from a server indexed in the SPF record for the domain, it's discarded. Thus nobody will be able to mask an e-mail address to make it look as if you are e-mailing spam messages. This method is also referred to as email spoofing.
