Email Forwarding / Spam Filtering on Windows Servers

I am looking to start up a web project that would involve using a Windows 2000 (or maybe 2003) web server. Unfortunately, I have only used Linux boxes for all my previous hosting needs, so I am not familiar with how email works for Windows... so I have a couple of questions.

Requirement:
- What I am looking to do is create a website with the feature of providing each user with an email forwarding address. (ie. user@mysite.com will forward to user@their_email.com). It is imperitive that I can automate the process for creating these new email forwarding accounts.

If I were to do this on a linux box, I would simply write out a .qmail files (.qmail-mydomain:com-user) that has "&user@their_email.com" as the only line... However, if I'm on a Windows box, I'd assume that I need to have a seperate Mail management system.

1) Is there an 'easy' way of handling the forwarding of email on Windows 2000? ie. Is there a way to use the SMTP functionality? Could you point me to a website that could provide me with more information?

2) I would like to avoid using Microsoft Exchange for several reasons: a) Expensive b) Not that stable / reliable unless you throw LOTS of hardware at it... (ie. I can't afford it...) c)Expensive

3) Are there any third party Email servers that others would recomend? My requirements are:
a) Ability to automatically (using calls from ASP.net code) create a forwarding email address.
b) Ability to filter for SPAM / Virus'
c) Scalibility (5,000 to 20,000+ email addresses)
d) Support for POP or IMAP accounts.

I have found MailMax 5.5 mentioned on one website - but their website isn't detailed enough for me to know if it would satify requirement 3a. (Automatic forward account generation).

Any suggestions / insights / direction would be greatly appreciated.

Thanks.
Dave