In my experience, we got put on RBLs in a heartbeat from the mailings (a list typically had 500-5000 addresses), though most of them weren't big enough to cause real problems. We never tried throttling - we were creating and marketing events, and couldn't afford to slow them down.

A board I run has similar problems. We're a modest site (anywhere from 5,000 to 25,000 posts a month, with about a thousand active members) and we share a server with a score or so of other sites. I'm forever working with Hotmail/Live and BellSouth (but oddly never the rest of ATT, at least so far) to allow our modest number of messages through - mostly registrations, notifications of PMs, and thread subscriptions. My hosting provider claims none of the sites do bulk email, but I'm not sure (otherwise, they're solid). But still, once a quarter or so I'm jumping through hoops for the big guys.

But first things first - check the Terms of your ISP and see what's allowed. Many of them ban bulk mail, though I don't know how - or even if - they look for it. I do suspect my two problem ISPs have bots that spot and act on the repeat traffic from our board.

-jk