Now if I use the mail program and yahoo mail it does NOT distinguish"The mail program" - I'm not an iPhone user, is that referring to a built-in app?
One thing you could try is a different email app. I think the Gmail app is available for iPhone, and it works with any email service that provides standard IMAP and SMTP connections.
Exactly the same issue my husband has usingSo, "the highest rated email app on the market" doesn't even render blockquote legibly? Sorry, I couldn't resist taking a snarky jab at those annoying ads Yahoo's been putting on Y!Groups messages lately.
In terms of what you can do on your own, I'd recommend going to the Gmail app - especially on your husband's Android. Unless there's something he didn't like about it that caused him to prefer the Yahoo app. The Gmail app has its warts too, so that would be understandable.
In terms of what Groups.io can do about it, I don't know. I'm no expert in the use of HTML across all the various email user interfaces out there, but I suspect that attempting to control the rendering via HTML style controls would be a "that way lies madness" situation. Testing it under so many interfaces is a daunting proposition.
Mark, on the other hand, may know of a silver bullet.