Hi Petri,
There is no built in way to block specific item counts in Exchange, as there could be legitimate reasons for having many items (like an archive mailbox). This script is more intended to alert administrators of potential problems, and allow them to take action. The best way to combat this would be to deploy MRM, and set policies to move items based on a certain date so that items do not pile up in user’s critical folders. Additionally, this may be a good time to educate users on how to manage their mailbox. There are also multiple performance counters built into Exchange to monitor slow searches, which could be caused as the result of having too many items. These could be monitored directly through Perfmon, or through a monitoring tool like SCOM.
http://technet.microsoft.com/en-us/library/bb201689(EXCHG.80).aspx