I switched to pine in ... 1993 ? I've never used anything else (except for brief stints of corporate email account and consulting, many years ago).
You'll note that in the "screenshots from famous developers"[1] that Brian Kernighan (Unix legend, the K in K&R and AWK) is using alpine in his 2015 screenshot...
[1] https://anders.unix.se/2015/12/10/screenshots-from-developer...
But I guess I don't see how you could turn that into a 'mobile' client. For the time I used pine I was ssh-ing into a box of mine for most of the day, while I was in front of a computer. I didn't care much about mail notifications on the go.
Now I do. If there's some (reliable! I don't care about the initial one-off effort) way to make that work with emails on the go, I'd be glad to learn about that.
Solution to this is to remember what the purpose of email actually is. It's supposed to be asynchronous communication, with the expectation that it may be checked, at most, 1-3 times daily. If you need communication immediately and in any location, instant messaging covers that use case much better.
For me mail is for async, coherent exchanges. IM is not a replacement: It is usually a conversation, usually short and short-lived and synchronous.
Async doesn't have to imply that I don't get a notification about your mail. It just means that I probably won't answer right away - or at least on my time.
Anyway: Mail plus ~instant~ notification is a thing for me and I don't want to give that up.
Mailpile (I'm a backer) might be interesting, but is still unstable and the future isn't certain for happy reasons (i.e. 'paternity leave'). Stumbled upon Whiteout from the references in the mailpile blog, only to learn that the company behind that effort is dead. No clue if or how this project will continue.
I regularly see (new) mail clients mocked as 'unnecessary', but I'm still waiting for a decent one. Mail as a medium works incredibly well for me, but it feels unpolished to use whereever I am.