It seems that it's better to stay on windows than changingig to unixoid system.
I had no Problem to set Up a "mercury Mail system", a "Mail@work" and a "hmailserver" as "passive" Mailsystem with several local mailboxes each assigned to a public e-mail adress withot a public MX Entry or MX Lookup.
sending mails via Relayhosts (according to senders public e-mail adress - sender based routing) retrieving mails via Pop3, supplying mails to the Cliants via IMAP.
In other Words:
To the internet (WAN) the Mail Server acts like a mail client with many Mailboxes
internal (LAN and VPN) the Mailserver acts as Mailserver to the clients
I found many "How To's" stetting up a mail server, but only one for nearly this behavior and this "how to" was for a Synology NAS (http://www.synology-wiki.de/index.php/Hauptseite) and though not well applicable for me. (I'm a german).