You do need Exchange CALs (Client Access License) in addition to your normal server CALs, but the answer to how many you need depends on how you are set up for licensing (‘per user’ or ‘per device’).
One of the questions here was remotely similar:
Q. If several users share the same desktop, do they each need an Exchange Enterprise CAL?
A. Customers may still license Exchange Server 2010 with either per-user or per-device CALs. If several users share a desktop and do not individually access the Exchange server from other locations (e.g. their home PCs) then they can be licensed with just one device CAL.