MicrosoftI've heard good things about Fastmail and Thunderbird. I've tried both out and they both allow me to move my mail to local folders if I prefer not to keep everything in the cloud. The current Thunderbird version I tried (128.2.0esr) DOES have a decent calendar function and Mozilla is working on a Thunderbird Android mail client, too (https://blog.thunderbird.net/2024/07/thunderbird-for-android-k-9-mail-june-2024-progress-report/). I've heard good things about Protonmail too, although I haven't yet tried it.
I'm still keeping my eyes open for other mail clients, but so far those two are the best alternatives to Outlook that I've found to be acceptable, even though they're not complete replacements for everything I'm used to with Outlook.
My preference, if that wasn't already obvious, is that Outlook desktop will continue to be supported, with the cloud version as an alternative for those who might want it.
