Deleting Default Mailbox Database in MS Exchange 2016
If you try to remove the default mailbox database in Exchange 2016 EAC (Exchange Admin Center) you may get following error:
This mailbox database contains one or more mailboxes, mailbox plans, archive mailboxes, public folder mailboxes or arbitration mailboxes, Audit mailboxes, To get a list of almailboxes in this database, run the command Get-Mailbox -Database <Database ID>. To get a list of ail mailbox plans in this database, run the command Get-MailboxPlan...
This is because the default database contains number of system mailboxes that are normally not visible in GUI. For example arbitration, monitoring, AuditLog mailboxes.