If your Exchange Server Event Viewer contains:
LicenseService, Event ID 201,
Error Description = No License was available for user <domain>\<account> using product MSExchangeIS
then you are out of Exchange CALs.
To correct the problem:
1. Buy additional Exchange CALs from JSI, Inc..
2. Start / Network Administration / Windows NT License Manager / Products View / MSExchangeIS
3. Press License / New License.
4. Enter the number of licenses purchased.
5. Click OK.
6. Check the box and press OK.
7. Shutdown and restart the server.