Sender Policy Framework, or SPF, is a validation system, that is used to prevent the so-called e-mail spoofing where an email message can be sent from one email, but to appear as being sent from another one, generally with the objective to fraud the recipient someway. If SPF protection is activated for a domain name, an exclusive record is generated for it in the Domain Name System and all the DNS servers around the world have it. The record includes all the e-mail servers that are permitted to send valid messages from an address under the domain. When a message is sent, the very first DNS server it encounters checks the SPF record and in case the sending server is permitted, the message is sent to the targeted destination. In case, however, the sending server is not present in the SPF record for the given domain, the email message won't be submitted and it will be discarded. In case you employ this solution, it will stop third parties from sending spam emails that look as if they have been sent by you.