I deleted a database on our Exchange 2013 (after moving all user, arbitration and public folder mailboxes to another database) and while doing that I got an error that said it was unable to delete a monitoring mailbox. If I now run 'Get-Mailbox -monitor' the results include three Health Monitoring Mailboxes that don't belong to any database.

1. When deleting the database (via EAC) I got the following message:

"Failed to remove monitoring mailbox object of database "Default". Exception: Active Directory operation failed on dc.contoso.local. This error is not retriable. Additional information: Access is denied. Active directory response: 00000005: SecErr: DSID-031520B2, problem 4003 (INSUFF_ACCESS_RIGHTS), data 0 .

Do you see the monitoring mailboxes for the new database? I have seen this issue as well - orphaned objects after deleting the database.

No guarantee, this will work for you but what I wound up doing is simply deleting the AD objects under the MESO (Microsoft Exchange System Objects ) folder referenced in those errors and restarting the Exchange Health Service on the server for good measure. Make sure you only delete the invalid objects.  

Yes, I do see two monitoring mailboxes for each of the other databases we have (3 databases, six mailboxes in total) just as expected. I read somewhere that two is the magic number, so I find it a little peculiar that we got 3 orphaned ones when deleting that database.

I guess I'll try your idea and see what happens. Thanks!

Well, if it makes you feel any better, I have 3 as well on the original 2013 database .

If you pipe it to :

get-mailbox -Monitoring | Get-MailboxStatistics

You will probably see that one of those health mailboxes has not been logged into in awhile. 

After deleting the three orphaned mailboxes and restarting Microsoft Exchange Health Manager servce everything looks OK when running get-mailbox -Monitoring. I'm just a little unsure what the consequences of this might be, but I guess we'll see that later on :)

Thanks Andy!