The way Kmail shows the content of previous mails is not always the same. The way e.g. Thunderbird does this is very much so! Always it shows colored vertical lines and even within the quotations automatic line-wrap works!
I have tried to make Kmail act in a similar way but I can't find it. Does somebody know how to make kmail show previous mail content in a consistent and elegant way or is it not possible?

It's not very clear what you mean by "previous mail". Do you have a screenshot or something that shows what you mean?

Thank you for you reaction Einar,

What I mean is this:

One time Kmail displays the content of the earlier mails of the treat as follows:
the text is green with these >> signs



But sometimes kmail shows earlier mails like this:



or like this: (à la Thunderbird)
there seems to be no logic...

I'm thinking it has to do with how mails are represented (HTML or text): I never noticed, as I only use text. Should you want a better look, please file a wishlist request on bugs.kde.org. I'm fairly sure this can be implemented.

Thank you Einar.