And greylisting can be a problem as well. There are some broken mail
systems (sending legitimate mail) which treat the greylister's
"temporary failure, please try later" as a delivery acceptance and
never resend. Nor do they (or the particular one I have encountered)
give a non-delivery report to the sender.
I would guess that the people who object to CBV will also object to
greylisting. It's yet another form of cost-shifting -- you're forcing the
sending server to keep the message on its queue longer and make more
attempts to try to send it.