As Krishopper said, unless you know what your mail volumes are going to be, it's hard to spec this out for you.
Also, are you going to allow mobile access? IMAP clients (which are very resource intensive)?
If you don't know or can't get this info, then you'll need to decide whether to have a system that may be "light" on resources, with user slowdowns -- at which point you can add more servers; OR, if you have the budget for a beefier system which might be significantly underutilized.
If you are migrating these users from an existing system, the data for which we are asking should be available I would think.
Hope that helps,
Mark
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