Exchange Server 2013 Unknown System Mailbox

Hello friends

We have Ex2013. On ExMgmtShell, first, I ran "get-mailbox" command I got this result:

[PS] C:\Windows\system32>$say=get-mailbox -ResultSize unlimited | measure
WARNING: The object kkk.tsk.mil.tr/Users/SystemMailbox{5B88CFDB -************} has been corrupted, and
it's in an inconsistent state. The following validation errors happened:
WARNING: Database is mandatory on UserMailbox.
WARNING: Database is mandatory on UserMailbox.

Afterwards, I ran "get-mailbox -arbitration" command, the SystemMailbox didn't show up.

Finally I googled default Exchange 2013 Arbitration Mailbox and could not find a mailbox named like SystemMailbox{5B88CFDB -************}

I know how to fix the arbitration mailboxes. The question in my mind is "what is this System Mailbox, how it is created and what it is for?"

Does anyone has any opinion?