Let me add that I'm now using the 'M' keyboard shortcut, that toggles between the read and unread states. It's a perfectly viable solution, whilst waiting for the UI in the cards.
I agree, the button to mark read/unread should be there.
I like the less cluttered approach of the "cards view". Take a couple-three lines to show subject, correspondent, date. But also include the needed button.
Table view (classic view) has the possibility to quickly change read status, via the sort column for read status. (It is a green bullet that is on or off.) I use this very often to set my mails to read, sometimes to unread.
YES. This and also the trash button. Basically anything that's available in tables, should also be available in cards. I would love to at least try out cards view, but without being able to have the options I need for productivity, it's not something I'd enjoy using.
Additionally, in tables, the unread indicator is a satisfying shade of green, or grey if read. In cards, if read, the indicator simply does not exist, and if unread, it's a very jarring yellow colour. If the unread indicator could be added to cards (and changed to green like in tables), along with the delete button, I would switch to cards view immediately. As of now however, it's a dealbreaker.
Please add not just the sender but the option of showing all correspondents in the Card View. This was possible in the list view. As someone who stores sent and received emails in the same folder this is crucial since otherwise I appear on all my sent emails instead of my recipients.