Run this command first\nSet-AdServerSettings -ViewEntireForest $True\nThe first thing you must do is move all the mailboxes to another database. This can be accomplished via this command\nget-mailbox –database "Database You're trying to remove" | new-MoveRequest –targetdatabase "Database to move users to"\nNow you must move the arbitration mailbox\nget-mailbox –arbitration –database "Database You're trying to remove" | new-MoveRequest –targetdatabase "Database to move users to"\nYou can now check the status of the mailbox move\nget-moverequest\nOnce they are all completed run this command to remove the request\nget-moverequest | remove-moverequest\nYou can now remove your mailbox\nRemove-mailboxdatabase –id "Database You're trying to remove"