using add-mailboxfolderpermission operation couldn't be performed

I like to use Exchange 2010 Powershell to give review rights for user B into the the calendar of user B. Seems easy, but no I ran into errors.

I use: Add-mailboxfolderpermission -identity UserA@domain.com:\calendar-User  UserB@domain.com -AccessRights Reviewer

I get the error: The operation couldn't be performed because userA@domain.com:\calendar couldn't be found.

I tried several users, same error. Strangely enough a meeting room box did work.

What am i doing wrong?