Reading tip for this weekend: "How to select the right hardware components for your groupware infrastructure" (PDF) http://t.co/2umNvaEet3
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You can get an Archiver license for as many users as you want, but minimum 20. The amount of Archiver users does not have to be equal to the amount of Zarafa users, but it must be less. So if you have a Zarafa license for 100 users, you can get an Archiver license for anywhere between 20 and 100 users (in multiples of 5).