Chris,
You're between a rock and a hard place. If you simultaneously believe
that all of the following are sinful:
1. To accept and deliver forged mail.
2. To accept forged mail and silently discard it.
3. To accept forged mail and then generate a bounce.
4. To reject forged mail because the sender might generate a bounce.
then you have no alternatives left.
The current drafts give you a way to avoid (1) -- that's their whole
point. They leave it as a local policy decision as to which of 2, 3 or
4 you choose. They recommend (4) with a "SHOULD", but you can
reasonably do either of the others.
Today, many large ISPs silently discard mail that they classify as spam.
I expect that those ISPs will choose (2) when implementing SenderID.
Your thoughts about MAIL-FROM, HELO and EHLO don't change the above
calculus at all. They merely change the details of which messages you
classify as forged.
-- Jim Lyon