I’ve solved this, but it’s a “heads-up” for others.
I’ve written before about the fact that eM Client cannot receive mail properly from Apple Mail (line breaks and signatures cause text to vanish or arrive as mysterious, undefined attachments in either txt or html, etc.) When I’ve complained to eM Client TS, they tell me to go yell at Apple, which conveys a bad attitude, is a ridiculous reply, and totally non-constructive, but whatever.
The new wrinkle is that iOS 11 automatically turns on what Apple calls “smart punctuation”, meaning that it replaces things like straight quotes and makes them more “typographical”. Problem is, they’re non-standard.
So if someone has updated to iOS 11.x and mails you from their iPhone saying the following: I’m “Bat Man”.
You will get mail saying: I?fm ?gBat Man?h.
Congratulations on this, and good luck contacting Apple.