> It makes absolutely no sense to have the email content panel below the email list, because emails don't have that aspect ratio,

It does make sense, because the list of emails typically requires the full length of your window. And while most emails may not use the full width of the window - some do; and one shouldn't have to open a separate window for those.

    +--+---------------+
    | f| list of       |
    | o| folder emails,|
    | l| one per line  |
    | d+---------------+
    | e| selected email|
    | r| contents      |
    | s|               |
    +--+---------------+

is the only way to go... for me. I realize that some people like it different and that's fine.