Kmail, Thunderbird, Evolution. That's pretty much it.
There's always some weird niche client somewhere but it won't be a hidden gem. Although I guess you can always use Pine (or rather Alpine nowadays) if you want to appear ubergeeky.
I think you should just write it for Linux. Since Windows has that Linux Windows Thing (I don't remember what it's called), it'll probably work there as well as long as you mark it as a dependency.
That's why I only use totallylegitfileconverter.ru they even have a lottery where they can directly transfer the money to your bank account when you win. They're very friendly!
OTOH, I don't think we've ever liked any of our presidents. Their approval ratings are so abysmal that they would horryfy US politicians (where 40% is low). Macron is quite bad, but is he actually worse than the rest? We might end up regretting him when LePen gets elected in a couple years.
That's a real problem when you're used to Kde and have to use a windows machine.
(Why is this damn thing so slow ? Oooh, right, double click)