Sender Policy Framework, or SPF, is a validation system, that is intended to prevent the so-called email spoofing where an email message can be sent from one email, but to seem as being sent from another, frequently with the purpose to fraud the recipient for some reason. When SPF protection is activated for a domain name, a special record is generated for it in the Domain Name System and all DNS servers around the world receive it. The record contains all the mail servers that are allowed to send valid messages from an address within the domain. When an e-mail is sent, the first DNS server it goes to checks the SPF record and when its sending server is permitted, the message is sent to the targeted receiver. When, however, the sending server doesn't exist in the SPF record for the specified domain, the e-mail message will not be submitted and it'll be discarded. If you use this solution, it will prevent third parties from sending spam e-mails which look as if they have been sent by you.