When opening a delegated Maildatabase by clicking on the username (screenshot attached) of the owner of the delegated database, the "mail" view is opened per default.
The problem with this is that most of the users will delegate only the access to their calendar. So it makes more sense to change this behaviour so that the "calendar" view is opened per default when clicking on the username (owner of the database) of the delegated maildatabase.
Other option would be to check on the fly if the user has access to the "mail" view, if that is the case, open the "mail" view, if not, open the "calendar" view.
Opening the "mail" view of a database where the user doesn't have access to, results in an error....