My 0.02c...
It's *very* hard to mess with the SMTP protocol - it should be avoided
as much as possible, since only a miniscule fraction of email admins
will implement such an idea (not to mention - not all SMTP code is
going to even *have* updates to handle this)
We (programmers) are at the extreme end of the email spectrum - it
rarely matters if we miss a legit email. "Real People" (tm) are the
other end - missing even just one real email can mean life-and-death
(literal, or just business/reputation) to some of them, so creating
anything that is going to cause this should also be avoided.
Remember - you're NOT NOT NOT trying to combat spam. SPF is not going
to make the slightest teensy tad of difference to SPAM, so try and
stay focused on the entire point of SPF - sender validity - and get
rid of the idea that just because someone sent their "evacuate the
building - the israellis are about to bomb it" email from a net cafe
down the road instead of their regular PC, that it's a good idea to
trash it completely!!!
(Don't flame the net-cafe analogy - you *know* what I mean)
Chris.