Ben, great question (nothing like waiting 18 months for a reply :-).
I think you should be able to do this under Advanced Settings -> Abuse Detection.
But throttling inbound SMTP would still be a nice feature to have:
- keep a compromised account from overloading your spool
- slow down an external spammer from delivering thousands of messages to your users
- throttle high-volume senders (eBay, FaceBook, etc.) so they don't overload your server
Maybe change it from a Question to an Idea. You have my vote.
Kevin