When decommissioning an Exchange 2019 Mailbox server and receiving a warning about mailboxes still being present, moving the arbitration mailboxes to another server is the appropriate first step. Arbitration mailboxes are system mailboxes used for managing approval workflows, eDiscovery searches, and other administrative tasks. Their presence in the database will prevent its removal. Use the Move-Mailbox cmdlet to transfer these special mailboxes to another server.