I guess if you would ask Microsoft, you'd have to buy Exchange and use Outlook Web Access as webmail. In Exchange you would have to configure the different domains and mailboxes.
In IIS, if I recall correctly, you can add one (or more?) Virtual SMTP hosts, and define there what should happen to incoming mails. There are a number of open-source and/or free webmail solutions you could link to this. (I found this just with a quick google around.)
In both cases you'll have to confige MX entries in the DNS records for the domains. So the mail system knows where to send the mails to.