You do need Exchange CALs (Client Access License) in addition to your normal server CALs, but the answer to how <i>many</i> you need depends on how you are set up for licensing (‘per user’ or ‘per device’).
One of the questions on <a href=”http://www.microsoft.com/exchange/en-us/licensing-faq.aspx”>http://www.microsoft.com/exchange/en-us/licensing-faq.aspx</a> 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.