How many CALs do I need on Exchange 2003 if I have 100 email users. 90 of them email users via webmail (OWA) and 10 of them email users via outlook and VPN. I wonder if I need 100 CALs or only as much as CAL that users are online at the same time. For example, if the maximum numbers of the users online at the same time is only 20, then I need only 20 CALs. Your clear answer would be highly appreciated.

I would contact a licensed Microsoft sales rep so they can work with you and understand how many users/devices etc...are accessing your Exchange org.

Andy, what you recommended is the first easy solution. But the problem is here that a Microsoft Sales rep answer without thinking is "you need 100 CALs". You know why beacause they want to sell more CALs. Each CAL is about $60.00 I searched internet, some believe that I need 100 CALs for 100 users and some believe that I need only the maximum bumber of the users at the same time connected to server. The question is still there. How many CAL do I need? Microsoft Sales rep is not a good source for answer, I think their answer is more economically than technically :)

The problem is there is different ways to license and no one here can give you the correct answer :) All I can do is point you to the official documentation: http://www.microsoft.com/india/sam/lic_cal.mspx