Hi Henrik,
1) dead easy to install. I use the free version. Just turn off any SMTP service in IIS before you start.
You can set up as many domains as you like with their own mailboxes. Each domain will have a "mail server
" address of Mail.this domain.com or Mail.that domain.co.uk etc.
With 1and1 pointing was easy, I can change the default mail server in my control pane. It's as easy as pointing each domain's mail server to your static IP address.
The free version isn't supposed to have IMAP - but it does. I have my iPad set to access mail via the IMAP facility.
Webmail, don't use it myself but one time when the wheels fell off my Outlook on a PC, I used Horde - which came free with my server package. Its pretty crap but it worked. Mail enable itself doesn't have a webmail interface built in. (Well at least the free version doesn't.
Oh yes, windows server 2008 web edition.
Ps ... Never trust end users. "I never got your mail" is a standard response when they haven't done something theynarensupposed to " when I wore a younger mans clothes it used to be "the cheque's in the post"