This article is about encrypting the body of the email which is easy* but no widely implemented standard exists.
* Stupid easy for two nerds to email securely.
* Stupid hard to work with multiple people and non-nerds.
So there are mechanisms to put encrypted things in workplace emails and then have some mechanism for receiver in a different organization to unencrypt. I have seen a mechanism that comes down to magic links, which I found ironic (though yes, intercepting is less of a threat than sending the data unencrypted).
I feel like supporting an option to not send an email unless STARTTLS happens is the way to go. There’s probably a lot of practical problems for, say, online Outlook or Gmail supporting that option when sending an email. But I feel like that’s the easiest solution.