Quick and easy MS licensing question

I know this is a "n00b" type question, but bear with me.. I'm having a brainfreeze or something :)

I forget exactly how the licensing works for Windows Server 2003... if I have a server that's going to run Exchange 2003, do I need CALs for both Server 2003 AND Exchange 2003?

e.g. if my org has 40 users (lets assume I'm *not* using Small Biz Server), I would need 40 CALs for Server 2003 (~$1,600) plus the cost of Server 2003 (~$1,200), plus the cost of EXCHANGE 2003 (~$700), PLUS 40 Exchange licenses (~$2,700). Essentially having to pay twice for licenses (once to access Server 2003, once to be able to use Exchange)? Or would I only need the Exchange licenses since employees are *only* going to be accessing the server for Exchange?