Mutt. Seriously.

(Other software has bugs. Mutt has fleas.)

See also: Neomutt.

Alpine. Probably fits your requirement for "simpler", though mutt, as with vim, strongly rewards familiarity.

Gnus or mu4e, under Emacs.

Old school: Elm, pine.

See: <https://en.wikipedia.org/wiki/Text-based_email_client>.

I was going to suggest mutt, but seems you've ruled it out? I'm curious why.

Have you tried Alpine?