During the process of mailboxes server Exchange 2010/2013 installation mailbox default database is created. If you try to delete default database you will get an error. The error appears because service mailboxes presence in the database.
Service mailboxes transfer
For database deletion it is necessary to transfer these mailboxes to the another database. The following command is needed to be performed for arbitration mailboxes search:
Get-Mailbox -Arbitration -Database "Mailbox Database 1200388344"
Now let’s move these mailboxes into another database:
Get-Mailbox -Arbitration -Database "Mailbox Database 1200388344" | New-MoveRequest -TargetDatabase prmlabmbx01-prmdag01-db01
Except for arbitration mailboxes in Default Database there are also administrator Mailbox and Discovery Search Mailbox. They also should be transfered.
Get-Mailbox -Database "Mailbox Database 1200388344" | New-MoveRequest -TargetDatabase prmlabmbx01-prmdag01-db01
Let’s check if transfer was successful.
After transfer the default database may be deleted.
Remove-MailboxDatabase "Mailbox Database 1200388344"