Exchange Licensing

I;m quite confused by Exchange 2003 licensing - do you buy a license for the exchange server & then every email address needs a license too (is that was a CAL is?)

For example - lets say I have exchange with 3 email domains

Domain1 has say 10 email boxes
Domain 2 has 5 email boxes
Domain 3 has 2 email boxes

So - that is 17 email addresses - does that mean I need 17 licenses in addition to the server license?

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