Years ago there was a Mac mail client called Mailsmith that was all about plain text email. It was mutt’s spiritual cousin, but with a GUI. This morning I am wishing it had survived to become an iOS client. There are none, apparently, that can reliably send plaintext mail.
I don’t want to be too dewey-eyed about poor Mailsmith, though. It was for most of its life just a POP3 client, and if I had one point of huge contention with Mac orthodoxy back then, it was the whole “POP3 is enough for anyone” thing. It took Apple realizing POP would make email a misery for all of its iPhone mail users and sensibly embracing IMAP before that orthodoxy could be broken.
I think Mailsmith tried to do IMAP in the end, but it read between the lines like there was no money in email clients, and no money in untangling innards too wrapped around the relatively straightforward “just download all the mail and don’t worry about other clients hitting the same inbox” charms of POP at its simplest.
I don’t want to care about all that, but do, and I really, really wish there was at least a magic button to press in Mail.app that just preserved attribution marks and made replies plain text.