Thunderbird also seems to be facing a massive rewrite.[0] Are there any good FLOSS, local (with a local mail store, if desired), cross-platform, secure (with security protocols implemented) email clients that support standard features and have a reliable future? And any of those that I could give to non-technical end users?

I know the names of a few, but I haven't tried them: Sylpheed, Claws, Evolution (cross-platform?), Kmail (cross-platform?), N1 by Nylon, MailPile (FLOSS?) ...

[0] See this discussion, for example: https://mail.mozilla.org/pipermail/tb-planning/2017-March/00...