Likely the easiest way to monitor for this condition would be to alert on the event generated in the Windows Event Log when this occurs. Another possibility is to see if the Windows Performance Counters for the mail store disappear when it's dismounted. In that case you could simply monitor any performance counter related to that mail store and alert when it's in a down or unknown status.
However, probably the best way to monitor the status of the mailbox store is the same way your users do. Using the MAPI, POP3, or IMAP4 User Experience Monitors you can monitor any mailbox in that mail store to verify that mail is both being received and delivered correctly.
