Why did SPF cause my mail to be rejected?
And what is SPF..
SPF is an extension to Internet e-mail. It prevents unauthorized people from forging your e-mail address. But for it to work, your own or your e-mail service provider's setup may need to be adjusted. Otherwise, the system may mistake you for an unauthorized sender.
Note that there is no central institution that enforces SPF. If a message of yours gets blocked due to SPF, this is because,
(1) your domain has declared an SPF policy that forbids you to send through the mail server through which you sent the message,
(2) the recipient's mail server detected this and blocked the message.