after opening the message explicitly (for example by double-clicking the message). To me, this is the most natural option when browsing mails, but it is missing.
Automatically marking a message as read only when it is explicitly opened - not (partly) previewed - is the only really sensible way of doing it, surely?
A message is only likely fully read is when it is opened in its 'fullness'; preview is only a 'letter-box' viewing of what the full content may contain - it's not really examined.