And if anyone is aware of Matrix integration in a traditional email client at this point, I'd be grateful to know of it. Seems like it ought to be an opportunity to lure non-technical users into using something with better security potential than email while staying with the letter as a metaphor and a familiar interface.

There is support in Mozilla Thunderbird - but as a chat mode, rather than email as you might be expecting. You can enable it by changing the value of "chat.prpls.prpl-matrix.disable" to false in the advanced config editor. Still, since it really just enables a chat client, another Matrix client might be better suited.

Remember when Google supported XMPP and Gmail auto-logged you into the Google Chat thingy? Was not that popular...

Email is a good analogy - this is even alluded to in the project FAQ: https://matrix.org/docs/guides/faq#how-is-this-like-e-mail%3...