Hi, Your best bet for answers to your question is to go to the Opera forum. You can get there through he Opera Bookmark list.
I don't understand why there is a need to mark it as read. My unopened mail is red and my opened is black so it is easy to me which I have opened.
I just delete them after I have finished using them rather than marking them as read other wise if I want to keep it I copy it to work.
My Opera does not read HTML mail but I believe this can be changed by going to preferences.
I learned a lot about Opera from their forum and suggest you try it. Peace Mal