I started with Thunderbird.
I found a problem -- later identified as being server-side -- and tried as many others as I could find in the distro's repos: Evolution, Sylpheed, Claws, KMail, Balsa, GNUstep Mail.app, Geary, and more.
Evolution is better than it was and isn't quite so determinedly Outlook-like any more. (I do not like Outlook.)
Claws is pretty good, but it isn't multithreaded, so it hangs when collecting mail. This is very annoying.
Claws and Sylpheed desperately need to merge again. They are basically the same app, but with slightly different feature sets. AIUI the author of Sylpheed, Yamamoto Hiroyuki, refuses to accept patches/PRs. He really needs to get over himself and learn to act a bit more like Linus Torvalds did. This intransigence is crippling both programs.
It is the 21st century and I do not want a CLI/text-mode email app. They have their place, for instance if you need to do email over ssh. I do not. But I want something that readily scales to a large window, has a CUA UI, can show basic formatting, etc. So, no Mutt/Neomutt/Pine for me.
In the end, I went back to Thunderbird and I still use it today. It is, after considerable research and experimentation, the best FOSS email app there is.
It is cross-platform: I can and do use the same app on Linux, Windows and macOS.
It talks to everything. I have or have had it connecting to Gmail, Hotmail, Yahoo, AOL, Exchange Server, Groupwise, CIX, and more different accounts and servers than I can remember.
It does address books and calendaring as well.
It has integration with handy features like Google's various chat and note-taking services.
It uses standard storage formats that can be accessed from other apps.
It's big, it is a bit sluggish, and like Firefox Quantum, some add-ons no longer work. This is a foolish decision of Mozilla's. However, it still has a useful range of add-ons.
It handles secure email and encryption well.
Snags: it really needs a working sync function.
But, after a lot of time and effort, it remains best-of-breed for my needs.
I needed more integrations for calendar and contacts with my self-hosted Nextcloud. I had to leverage plug-ins for Thunderbird and they were problematic but kind of worked most of the time with caldav and carddav data sources.
Earlier this year, I reset my Thunderbird profile and tried the updated non-plugin setups and it has been great. I know the overall roadmap and updates have not be dramatic but I can say the stability and core functions have really improved.
You should at least give it a shot.
Nothing else comes close.
Discussion on HN: https://news.ycombinator.com/item?id=25222166