Sender Policy Framework, or SPF, is a validation system, which is used to prevent the so-called email spoofing where an email message can be sent from one address, but to appear as being sent from a different one, generally with the objective to fraud the recipient for some reason. If SPF protection is activated for a domain, an exclusive record is generated for it in the Domain Name System and all the DNS servers worldwide receive it. The record contains all e-mail servers that are allowed to send authentic messages from an email address under the domain. When an e-mail is sent, the first DNS server it goes to verifies the SPF record and if the sending server is permitted, the message is forwarded to the targeted destination. In case, however, the sending server is not contained in the SPF record for the specified domain, the message won't be forwarded and it will be removed. If you use this solution, it'll stop third parties from sending spam e-mails which seem to have been sent from you.