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I have used both K-mail and Thunderbird on a smaller machine where I didn't want all the Qt/KDE-dependencies and I think they are equally good. Evolution is also good, but I have never really tried it that much.

So if I am on a Linux machine I prefer KDE and K-mail. If I am on Windows I prefer Linux :) but Thunderbird would be what I used on Windows.

I am a KDE OpenSuse Linux kind of guy... for years now. =)
As of KDE5 it has been lightened up with the amount of bloat it loads and is quite spritely now.

:) I am just trying it out. I always had enormous computers so as to be able to work with 3D-graphics so bloat was never my main concern, but I did try to install LXDE on my eeepc 901 and I did miss KDE a lot. So I'll see when it return to me (had to go home early from holiday and left it for my daughter) if I can trim KDE/Plasma (whatever they call it these days) down.

Try out LXQT. I have it installed as a back up option if I bork my KDE Plasma install with an update... which has happened in the past. A very slim option with a low memory foot print.

Cool! I'll have a look. QT makes me feel safe and at home :)

QT rocks.