Sender Policy Framework, or SPF, is a verification system, that is designed to prevent the so-called email spoofing where an email message can be sent from one e-mail address, but to look as being sent from another, ordinarily with the purpose to scam the recipient in some way. If SPF protection is activated for a domain, a unique record is made for it in the Domain Name System and all the DNS servers globally get it. The record contains all the e-mail servers that are allowed to send valid messages from an email address under the domain. When an e-mail is sent, the very first DNS server it goes to verifies the SPF record and if its sending server is authorized, the message is forwarded to the targeted receiver. In case, however, the sending server is not present in the SPF record for the given domain, the e-mail message will not be submitted and it'll be discarded. In case you use this solution, it'll prevent third parties from sending spam e-mails that appear to have been sent from you.