lets see I license a mail server from you, but inorder for anyone to read the mail I also have to license them to read it... Its damn near criminal is what it is.. Its like buying a car but being told you need to pay the dealer for every passanger who you intend to drive around.
You obviously can't read. Exchange can be a POP3 server or a web host, with no limit on the number of mailboxes.
Server 2003 requires licenses for local logins, but not remote logins.
The cost per seat for Windows Server is such a trivial percentage of the overall cost of workstations that it hardly enters into the decision making process. No business is going to make a software decision based on saving $10 per year.