Sometime you can run into situation that you Arbitration mailbox got deleted, this can happen if you accidentally delete them from Active Directory or during migration for older version of exchange . To check if you any Arbitration mailbox :
Get-Mailbox -Arbitration
if you get none you will need to re-create them !
To re-create them you need exchange instillation media inserted or mounted into the server. Then execute :
<Virtual DVD drive letter>:\Setup.exe /PrepareAD /IAcceptExchangeServerLicenseTermsThen make sure they have created :
Get-Mailbox -Arbitration
Now you need to enable those mailbox, For each exchange version you have different default mailbox Arbtration :
Enable-Mailbox –Arbitration –Identity “FederatedEmail.4c1f4d8b-8179-4148-93bf-00a95fa1e042”
Enable-Mailbox –Arbitration –Identity “SystemMailbox{e0dc1c29-89c3-4034-b678-e6c29d823ed9}”
Enable-Mailbox –Arbitration –Identity “SystemMailbox{bb558c35-97f1-4cb9-8ff7-d53741dc928c}”
More information :
https://jacob.gardiner-moon.co.uk/2015/11/26/microsoft-exchange-2013-2016-recreate-arbitration-mailboxes/
https://docs.microsoft.com/en-us/exchange/architecture/mailbox-servers/recreate-arbitration-mailboxes?view=exchserver-2019