Today, the sender from a SPF enable domain will get
"554 5.7.1 Command rejected" if he or she sends from a
machine thats not allow to send mail from that domain
(if it uses the "-all" statement).
But it would be much better if it could be changed to
"554 5.7.1 Rejected due to the SPF-policy of
$domain_of_MAIL_FROM" or something simular.
That would make it easier for users to interpret the
error message and for them to understand that "SPF" has
something to do with it...