Why are graylists that horrible? All it does is require the sending server to retry 5 minutes later; I don't see how that would have any impact on a business unless they are in the habit of being on the phone with new customers and asking them to send an email at the same time.
Assuming the sending server does that. Maybe it takes a few hours. Maybe it doesn't. Small businesses can be a mess, and you can't say "well, your customers suck" when the client complains about how greylisting is working for him.
