I went looking, and Mac Mail.app will replace "..." quote marks with “.. " which is half-pregnant: it modified only one side of the pair.
This causes a text/plain to be sent which is detected by vi(m) as UTF-8. So I kind-of like Mac for making an outcome which is correctly typed for encoding, and I kind-of hate Mac for calling this text/plain but then modifying what I typed to uplift 7 bit ascii clean typing to a modern world. I guess text/plain is true: its text, it's just not petroglyphs.
-G