What exactly is your requirement? Do you want someone lese run your server? Or you can run your own server, also, who does your DNS, and if they let you do MX record etc, etc.
As far as your criteria list:
1. can work behind a dynamic IP
Email server doesn't have anything to do with this, it's your DNS provider that needs to handle this
2. run on Microsoft / IIS
Once again, email server doesn't care about IIS, you just need a email server that runs on windows (this will limit your option greatly)
3. not buggy / insecure
Window itself is buggy and insecure, you prolly need to worry more about your server and email relay (spam) than anything else
4. easily configurable
Email is not an easy thing to setup if you want it to be complicated, however, basic option should be relatively easy to setup
5. web interface
you need IMAP support on the server, and you can download and run free web client for email like the one Gregg suggested
Since this is javaranch, besides mercury, I suppose you can try JAMES by jakarta project.
Note: running a stable email server is not easy task, you need to consider
alot of area, whether you want to allow POP3, how relay is setup, whether you need SSL or not, disk space, quota, SPAM, etc.