Hi all,
Maybe this article would help to understand possible migration scenarios -
Moving from a Linux Mail Server and peer-to-peer network to Exchange Server 2007 (Part 1).
At the same time, it is very sad that employer doesn't understand obvious benefits from using FOSS mail system (like, Zimbra).
Anyway, if there is a middle-size organization then it is easy to compare costs of migration and licenses (windows servers, exchange servers, windows CALs, exchange CALs + anti-virus CALs, Outlook licenses). There could be indirect expenses too (new hardware, Software Assurance, IT-staff education, etc.). Regards to Exchange 2007 documentation, you need at least 4 servers (2 HT+CAS, 2 MBX) - physical or virtual.
Snork, I hope that information could be useful for you.
Regards,
Roman