Licenses on SBS 4.5 and Exchange 5.5

I am sort of new to Small Business Server 4.5 and Exchange 5.5. I keep getting an error in Outlook on a few client computers that says “Unable to display this folder, there are no additional Microsoft Exchange client licenses available.” I thought that if I ran out of Client Access Licenses that it would not let the client log on to the network. They can log in ok but get this error in Outlook and can not get their mail. Where do I go to find out how many client access licenses I have? Do I need seperate licenses for Exchange eventhough it came with small business server? Thanks