Yes, sent mails are now marked as read, but if I take a look on my Sent Folder via Outlook for Windows, this sent item looks strange. It is a editable item, where I can edit To-field, subject and body.
Second problem with EWS: I'm not able to open a shared mailbox. Outlook error message: You do not have permission to open this folder. In Outlook for windows I'm able to open this shared mailbox.