How SPF Works
You can think of SPF as reverse mail exchange (MX) records for your domain. Because MX records define which machine should receive email for a given domain, reverse MX indicates which machines should be originating email for a given domain.
There are two pieces to Sender Policy Framework: the originating side and the enforcement side. The originating domain owner who wants to protect his or her domain from having spammers use it to forge MAIL FROM simply publishes the appropriate DNS TXT records in SPF format. Recipients who enforce SPF (in order to reduce the amount of spam coming into their users’ mailboxes) look up those DNS TXT records.
The verification process goes like this. The IP address of the originating email server ...