So it defines the border of the *sender*, and then the
*receiver* border box has to do the check?
Yes. The receiver is not forced to check it, but if he
wants to do it, it must be at the first box on his side,
typically one of his MXs.
Maybe it should define the receiver border as well?
Sorry, I don't quite get this, the receiver border is
clear, the MXs (or the A without any MX). That part
of the equation was always clear.
It wasn't clear for the sender side. One predecessor
of SPF was RMX => same idea as MX, only for senders.
As soon as you have both borders defined you can check:
"always coming from US" - "always going to MX". So if
it came from Belize to Mexico it wasn't US. Bye, Frank