Do you use a CLI based email client?

I don't. Partly because it's CLI, and I don't enjoy the CLI enough to go out of my way for it, especially not with email, which(Unless you're doing some inadvisable kumar-asshole.sh stuff) is not really automatable.

But largely because I rarely use mail clients at all aside from gmail on my phone. So many jobs don't have their own mail servers, and I'm not particularly interested in running a personal one either.

I do, however, occasionally back up and clear out my old gmail messages. I might consider using a CLI client for that, if it was 5-minutes-or-less levels of easy to get started and write a script for. Otherwise I'd just use Thunderbird.

Not that I particularly like thunderbird, I'm really not a fan of the UI, but it works.

I do, but not on my "main" box. I chose mutt-ng. I have a portable device based around a Raspberry Pi Zero, and the 512Mb memory is at a premium, so mutt is just right. It also has loads of hooks for GPG etc so it's just what I need. I can't see myself using it as my daily-goto mail client though.

I use Heirlooom-mailx. It is good enough, and does some things that are useful to me, and the other programs usually have stuff that I do not want.