Tuta Mail for personal

Thunderbird/K-9 (mobile) for my work's gmail since the gmail website is garbage

Proton web and mobile client. Also thunderbird

I use eM, it has tons of options and the mail rules are next level.

https://www.emclient.com/

Thunderbird on my pc and the default ios app on my phone for my non proton mail email accounts I just use the proton mail website when I use my proton email

Icedove (Thunderbird) works well enough for me. Maybe the reason it's "old fashioned" is because it works well enough that it doesn't need to be changed that often.

In the proprietary software world we're used to UI's being redesigned on a regular basis for no user benefit.

I’ve been wanting something too! What I really want is something that interfaces natively with Exchange server as well as integrated inbox for multiple accounts.

The one product that exists is Blue Mail which is pretty nice except that half its functionality is broken. I’ve been in contact with their support multiple times over many months and eventually they just gave up. Its functionality is limited by arbitrary glitches and unknown limitations which they simply don’t want to bother fixing.

Thunderbird, k-9, and aerc