This could be usefull when moving users and you would like to free up some space.

# Permanently delete disconnected mailboxes.

List disconnected:
Get-MailboxStatistics -Server MAILSERVER | ?{$_.DisconnectDate -ne $null} |ft Displayname,Identity,Disconnectreason

Remove by using mailbox database and mailbox identity.
remove-storemailbox -Database MAILBOXDATABASE -Identity “8be4d91d-6380-4dcd-b5a4-d003f4ec3f14” -MailboxState Disabled