This problem occurs because the
quotaNotificationSchedule attribute and the
quotaNotificationStyle attribute on the store object are not set correctly by Exchange System Manager. To confirm this problem on a mailbox store policy, view these attributes on the mailbox store object in Active Directory. If these attributes are listed as "not set," notifications are not sent.
You can find the mailbox store object at the following location:
CN=[Mailbox Store Name],CN=[Storage Group Name],CN=InformationStore,CN=[Server Name],CN=Servers,CN=[Administrative Group Name],CN=Administrative Groups,CN=[Organization Name],CN=Microsoft Exchange,CN=Services,CN=Configuration,DC=[Domain Name],DC=com