First, there is no direct migration path from sendmail to Exchange. You don't give any indication of your present network environment, so I'll have to shoot from the hip.
What operating environment are you currently using? Do you already have aWin2K Active Directory (AD) domain implemented or will you be migrating from an NT4 domain environment to Win2K? If not, installing Win2K and AD is the very first requirement to even install Exchange 2000. Exchange 2000 relies totally on user information stored in AD to set up and publish mailboxes. Exchange 2000 also relies totally on the SMTP service in IIS to provide Internet mail.
What operating systems are users presently using? If you are exclusively using some flavor of Unix or Linux on the desktop and plan to continue to do so (vs. migrate to a Windows environment), then you also need to create user accounts in Win2K in order to access Exchange. The time factor involved may determin whether you switch everything at once or incrementaly.
What email clients are being used? If you are already using Outlook, the major part of your battle is already (transferring saved email) is already doable.
Since 200 users is not an insignificant number of people to have pissed at you if things go wrong, I would suggest the following plan:
(based on Windows 2000 and active directory working correctly, with all users having active, working user accounts)
1. Install Exchange and ensure all the pieces are working correctly.Make sure you add the correct MX records to DNS.
2. Create a single test case using an active user (will probably be either you or a volunteer). Ensure send, receive, routing, address creation, etc. work properly for this single user.
3. Create a larger test group (preferably volunteers) of 10-15 people and conduct the same tests. This larger group will help you rule out problems related to anomolies in aliases, addres
