Version:            (using KDE 4.2.90)

To avoid having windows open for each mail, we would provide an icon/shortcut in the message pane that would switch to fullscreen mail display mode. In practice, this would mean hiding the folder list, favorite folder list and message list panes, displaying the selected message over the whole kmail window. It would be similar to Okular's presentation mode, only there is no need to hide the window decorations (or there could be a preference for that even).

There would be an icon present that would terminate this fullscreen mode and possibly icons for moving back/forward between messages (although arrow keys should be enough for this and no extra icon is really needed).

I find that for certain projects I keep receiving long emails that I open in a new window so I can see a large enough part of the mail at once, then I close that window, open the next message and repeat the process. Opening the message in a new tab (which is another item on the wishlist) wouldn't really help as I would still close the tab once I am finished reading it.

Thunderbird kind of has this feature: clicking on a message replaces the message list & message preview panes (but not the folder list!) with a larger view of the message, but to get the message list back you have to click on its containing folder again (which is why the folder list does not disappera) and this is not very friendly (e.g. you may not remember which folder you were in, especially if you keep 'unprocessed' messages in the inbox and 'processed' message in subject-related folders). I think the proposed method would work much better.