Sender Policy Framework, or SPF, is a validation system, that is used to prevent the so-called email spoofing where an email message is sent from one e-mail address, but to look as being sent from a different one, generally with the objective to scam the recipient in some way. If SPF protection is activated for a domain, a specific record is created for it in the Domain Name System and all DNS servers globally receive it. The record contains all the mail servers that are allowed to send valid messages from an email address within the domain. When an e-mail is sent, the very first DNS server it encounters checks the SPF record and in case its sending server is authorized, the message is forwarded to the target destination. If, however, the sending server is not included in the SPF record for the specified domain, the e-mail message will not be submitted and it will be discarded. Whenever you use this solution, it'll prevent third parties from sending spam which look as if they have been sent by you.