How to move system mailboxes to a new mail database

Moving Arbitration Mailboxes (System Mailbox , FederatedEmail)

Run the following command in Echxange Management Shell to list all system mailboxes

Get-Mailbox -Arbitration -Database "Current Database name"

Run the following command to move those mailboxes to a new database

Get-Mailbox -Arbitration -Database "Current Database name" | New-MoveRequest -TargetDatabase "New Database Name"