The internal health monitoring system may report Unhealthy, and an error similar to the following is reported by SCOM:
This is usually caused by invalid HealthMailboxes (e.g. health mailboxes that no longer exist or have been moved because a database has changed).
You can attempt to resolve the situation, but it's probably more easier to simply let the Exchange Health Monitor re-create the HealthMailboxes.
net stop MSExchangeHM
net start MSExchangeHM