L.S., When reading old or new emails I open the first (in a separate window) and I use the k-key or j-key to read the successive mails after another. Except for the first one, the other mails stay in status new (red evelop) or unread (blue envelop), which they should not, because all of them are read this way. The k and j-key is used for opening next and previous message. Thanks for a reply, EvdS.
You, no doubt, have the option 'Mark message as read' set to 'only when opened in a new window, or replied to'. The first time you open a message in the separate message view the separate message view is a new window. When you use the 'next' button you are not opening the messages in a new window, but in the same, now old window. Note that the option says 'new window' and not 'separate message view'.