Sender Policy Framework, or SPF, is a validation system, which is used to prevent the so-called email spoofing where an e-mail message is sent from one email, but to appear as being sent from another one, generally with the purpose to scam the recipient in some way. When SPF protection is activated for a domain name, a special record is created for it in the Domain Name System and all the DNS servers around the world get it. The record contains all e-mail servers that are permitted to send legitimate messages from an address part of the domain. When an e-mail is sent, the very first DNS server it goes to checks the SPF record and when the sending server is authorized, the message is sent to the target receiver. In case, however, the sending server is not included in the SPF record for the given domain, the message won't be sent and it'll be discarded. In case you use this solution, it will stop third parties from sending spam e-mails which seem to have been sent by you.