I was recently trying to remove the last Exchange 2013 Exchange server having already migrated to Exchange Online and setup the Exchange 2016 recipient management server. I was getting an error that the last database couldn’t be removed because it still had mailboxes on it. Indeed there was one Public Folder mailbox even though the Public Folder had successfully migrated to Exchange Online.
When trying to remove the Public Folder mailbox it gives the error “No active public folder mailboxes were found. This happens when no public folder mailboxes are provisioned or they are provisioned in ‘HoldForMigration’ mode. If you’re not currently performing a migration, create a public folder mailbox.”
I was able to resolve this by running this command:
Remove-Mailbox -PublicFolder PublicFolderMailbox1 -Force
With the mailbox removed, I could remove the mailbox database and uninstall the Exchange 2013 server.