There’s some decent alternatives to Outlook, but not so FOSS. An example might be Mailspring. And if you won’t use Thunderbird then you might need to look at some webmail suites. The best ones to check out are Posteo.de (German), Mailfence.com (Brussels), Protonmail.com (Swiss) and Tuta.com (Germany).

I don’t know why you wouldn’t use Thunderbird. It’s been brilliant for me for years. Some recent updates have improved the aesthetics and included some requested functionality. The Thunderbird team have also been working with K9 to improve the Android app. It’s only going to get better moving forward People are waking up to corporate proprietary abuse and are looking for alternatives. Mozilla are stepping up.