Encounter error when run Set-MailboxFolderPermission powershell command

Hi,

I need some advises.

I ran below script to change the room calendar permission for "default" from availability only to reviewer. 

Set-MailboxFolderPermission -Identity "xxxxx@xxxx.com:\calendar" -User default -AccessRights reviewer -DomainController xxxxx

But exchange management shell prompted The specified mailbox xxxxx@xxxx.com doesn't exist. I did tried to change the identity from email to domain to distinguish name, anyhow it still prompted the same error message.

Thanks.