Sender Policy Framework (SPF) is an email validation system designed to prevent email spam by addressing a common vulnerability, source address spoofing. SPF allows you to specify which hosts are allowed to send email from a given domain by creating a specific DNS SPF record in the public DNS. Mail exchangers then use the SPF DNS records to check that mail from a given domain is being sent by a host sanctioned by that domain's administrators.
Enable SPF checking by checking the box in front of it, then use the drop-down menus in order to select the actions to be taken if no SPF records are published or if SPF records cannot be checked. Choose between allowing to deliver the message or deleting the message.