Windows 10 Mail and Calendar
Not a bad email client. Snappy, works with multiple accounts. I did have trouble getting it to sync my contacts and calendars with Nextcloud*, but the core email features work well. So of course it’s being discontinued.
Rather than switching to “The New Outlook” (even if it is supposed to be more like the Mac and web versions than the Office 2019 version for Windows), I’m switching to Thunderbird. I already know it works with my setup, and I know it’s not going to constantly try to upsell me on Microsoft 365.
* Windows 10 doesn’t have a good way to sync with a Nextcloud instance directly, but you can work around it by starting to create a bogus iCloud account as a placeholder, then go into the advanced settings and paste in the CardDAV or CalDAV URL for your server. This is actually Nextcloud’s documented solution!
Also, for troubleshooting purposes: Windows Mail re-encodes messages when you export them. I had to troubleshoot a 7bit vs quoted-printable issue and had to use Thunderbird to see the actual code being sent.