What I need is letting users create their email accounts using web...just like yahoo and hotmail systems.

Was using socketmail on my Linux server to provide email service for my users. Had some issues in moving socketmail to Windows and unfortunately socketmail support does not respond for few months already.

Now that I moved to Windows box, I am looking for a similar system to be working on Windows.

I am running IIS and mysql database and MailEnable for my personal email.

Any recommendations?

If your willing to build your own, HMailserver offers an API that you could plug into to build that type of system.