Mailserver for Win32 or Win64

I have to work to construct a new server based on XP64. I have an existing Exchange 5.5, but it has proved a little too old.

I have two requirements for the mail server:

1. It must be able to handle multiple domains. (Or be able to run in several instances.)
2. Webmail option.

What will you recommend? I am willing to shell a little money on a slate license.

I use hmailserver, which worked brilliantly. It is easy to configure.

Unfortunately, there is webmail, but dont know how one can download one of the other perfect webmail available and apply in IIS.

Sendmail or postfix... they run well both well in Windows?

@ Steve123
hmm ... Then I have to start to compile more. Thank you anyways.

@ Spyrus
It looks interesting though.