I've been a Thunderbird user for some time now. I like how it behaves how I expect it to and does what it is told. I like being able to set all plain text and full headers all the time. I like its IMAP setup and how it works with gmail.
It is pretty plain looking but it behaves like a traditional email client and does not screw everything up by trying to make things "easier" for you.
I have not used much else other than web interfaces lately. I was an Outlook express user until they kept moving the plain text setting with every version I was never sure if the setting was still available. It has been in the same place in Thunderbird for ages now.
Thunderbird is to Outlook Express now, as KDE is to gnome. Know what I mean?
I don't like the look of the Windows Mail client from what I have seen. Far too "easy" and "friendly" and shiny and just begging for vulnerabilities... But to be fair, I have not really explored it at all - you might be able to set it to all plain text and full headers, but I doubt it.
I guess the answer to your question is what are you looking for? A traditional email cllient of the last twenty years? Or a new all web 2.Oy, iGottahavitpod, shiny rounded friendly pastel coloured - exploit me - pukefest? (lol, I guess my bias is not too obvious...
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