SPF - Sender Policy Framework - is broken and must Die
From Computer Tyme Support Wiki
SPF (Sender policy Framework) Sucks
SPF was a noble attempt to control spam. But it is a failed attempt and is being kept alive by openspf.org. The theory was that if you could tell the world which servers were allowed to send email for a domain then all other servers sending email for that domain could be rejected. The idea was the this information could be sent over DNS and it would be easy to implement. But in practice it wasn't that easy.