Reputation (or Lack Thereof) and Hotmail Rate Limiting

As I've disclosed before, I tend to spend a lot of time with companies discussing deliverability woes to certain ISPs. Today, I had the occasion to do the same with a pretty high profile sender whose test campaigns were being let's say "improperly manipulated" by the finicky Hotmail MX machines.

Per my last posting, I stated my opinion that most rate limiting is not due to not having the right technical specs the edge gateway is looking for, but rather it's a result of the system taking a cautious approach to mail coming in from an "unknown" sender. This case in question seems to confirm my supposition.

This mailer was sending a six-figure campaign from a fairly new IP address that definitely appeared to trip a rate limit. In fact, the logs indicated that Hotmail would only accept 5,000 messages one hour, do the same for the next hour, not accept ANY mail the next, and then would finally return a level 500 (permanent) error causing their MTA to bounce all remaining 20,000 messages or so from the queue. The one question our team had is that if the sender were to have self-rate limited the campaign, would Hotmail have let mail in the one hour where they blocked it altogether?

In any case, this might be a scenario where one can use empirical data to surmise the amount of mail a major ISP will let in without fully validating the "street cred" of the IP in question. If any marketers want to test this and get back to me with a report as to whether or not the same behavior occurs, please do!