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Thunderbird has the most features on offer thanks to all of the plugins available. I keep checking for other options, but I still have not yet found anything that can match it.

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.

Can't remember when I had a problem with it. I have used it with GPG too and that worked fine. But I am not really encrypting anything these days. Not many that know how to use GPG, and when people open the mail in their Gmail inbox my words are datamined by Google anyways.