Yeah, I throttle initial connections per IP to something like 15 or 20. I started doing this after I got hit with a little more than 600 connections/second for a few minutes.
Just a note to those who might not know - but Outlook (Express) and possibly other MUAs like to connect once per "account", so if you host multiple accounts for a single person, you'll have them connecting in once for each account whenever their client checks mail/starts up.
I host my own email and I have about 5 accounts (I've had more in the past) I check. The instant I set a throttle on connections per second I had tons of errors come up when I would check my mail since I couldn't successfully log in for all accounts.
What you'd possibly be more interested in if you're hosting mail for many people is some way to throttle based on account, though that would require peeking at the protocol data and such.
Eli.