When the license usage gets to within 80% of the total licenses registered the system sends an email (this in not configurable). The issue is that in situations where you have Active Directory integration an email is sent for each of the servers in defined OU mappings. This was identified as a defect previously and a hotfix was created to only send out 1 email per day.
Install Hotfix_20110411.zip which is available on the customer portal.
This has been address in version 2.4.804 as well, please refer to the release notes (BFER 2614).
The email message will be sent to sys-admins.