The Exchange Public Store storage quota must be limited.

This setting controls the maximum sizes of a public folder and the system’s response if these limits are exceeded. There are two available controls and the system response when the quota has been exceeded. The first control sends an email warning to Folder Owners roles, alerting them that the folder has exceeded its quota. The second level prevents posting any additional items to the folder. As a practical matter, Level 1 serves the purpose of prompting owners to manage their folders. Level 2 impedes users in their ability to work and is not required where folder use interruption is not acceptable. Public Folder Storage Quota Limitations are not a substitute for overall disk space monitoring.