New answers tagged intranet
You simply need a CAS and a mailbox server. It might be possible to install both roles on the same box. In Exchange 2016, the different roles are being abolished (or combined, if you prefer) so you will just need "an Exchange 2016 server" when that comes around.
In order for Exchange 2013 to work, you need at least one Mailbox server and one Client Access server; the two roles can be consolidated in the same server(s) or spread across multiple ones. The third available role, Edge Transport, is only used to handle Internet messages, thus it would be useless in your scenario.
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