That's what mutt is. You can compile it with support for imap and smtp, and most packaged versions of it are built that way, but the traditional way of using it is just as way of reading and sorting emails. Fetching emails from the server is done by a POP or IMAP agent (I use offlineimap), sending mail is handled by an smtp agent (I use msmtp), composing emails is done with $EDITOR.

I don't think that mutt is the most unix-philosophy-aligned MUA possible, it feels kind of like GNU tools: unixy, but jam packed with too many flags and options and convenience features. The unix philosophy would prefer that the application's design be changed to better accomodate implementing those extra features externally where plausible. But the perfect unix MUA would do the same things as mutt.