For some users, they're perfect (not all - schools etc need Win/Linux/Mac). I have family members who should just have a Chromebook as all they do is browse the internet - copy and paste is a stretch for them!
https://www.mailcoach.app/ Mailcoach by Spatie (who work on a lot of really good Laravel open source stuff) seems decent at this. You can either pay for a self-hosted license, or use their cloud.
Looking at this app, it's an absolute hidden gem of how amazing Android is as well.
The main app can have third party extensions provided just by installing more APKs. This is something I tried to explore quite a few years ago when I last had an Android phone, but seeing such a polished app which achieves it is nice.