Good cheap/free email server for my website?

I'm looking for a good cheap/free email server for my website, any recommendations?
Criteria:
can work behind a dynamic IP
run on Microsoft / IIS
not buggy / insecure
easily configurable
Optional:
web interface

Mercury is what I use. It is completely free. I haven't had any problems with it so far. I was running POP3 but just switched to IMAP.
There is no Web Interface, but I use SquirrelMail for my web interface.

I've been running Mailtraq (http://www.mailtraq.com) for about 5 years now.
Works well, has webmail support (version I have uses a built-in webserver, maybe the current version can plug into IIS).
Not free, but a lot cheaper than things like Lotus Notes and Microsoft Exchange.

Well mailtraq definately looks nice, but for now I'm trying to do this on the cheap, that means free if at all possible, and under $100 if not... Mailtraq pro (w/ webmail) for 5 users is $229...

I know the price, it's the only thing that keeps me from upgrading (let the upgrade offer expire, darn me).
It's still a lot lower than any comparable product I know though.

This might not be an option for you, but I have all 6 of my websites at http://www.zoneedit.com - they do the DNS stuff for me and I host the sites at on my own box at home. They do free email forwarding. I can setup new email accounts via the webinterface and I just have the mail forwarded to my ISP email account.
For you, you could keep your site hosted at the same place but use zoneedit's emai lforwarding feature.
Just a thought.
Brian

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.

I just wanted to run my own server for the experience, and so I could do just about anything I wanted to. So I ended up with mercury mail, which seems to do an ok job, though I wish I could be more confident in it, since it still looks and behaves like a win32 app

I would've gone with imail to begin with, but for some reason it DEMANDS a solid IP instead of a domain name, so that's why I had the requirement that it worked with behind a dynamic IP.
I tried that squirrel mail web client, but that never did the trick for me either, kept getting some strange login error, so I went with http://jwma.sf.net , which is a java web mail client, which almost works perfectly

But I'm not too worried about it, I've got everything up and mostly running, and if jwma doesn't improve in the next few months I'll just go ahead and try and code my own web email client