Do i have to purchase CAL’s for Exchange with Miscrosoft SBS?

Im in the process of setting up a network for a small to medium size organization that will support approx 50 users on 3 continents. Im trying to decide between MS Server 03 + exchange + CAL’s
and Microsoft SBS + IAS, or possibly setting up a linux mail server, which im reluctant to do because im pretty inexperienced with linux. Im confused about how the CALs work with SBS. Any help would be much appreciated.