So what do you do when the plain-text and html sections of an email disagree?
Tell the sender. If no one does, they won't know of this problem.
Of course, sometimes it happens that telling the sender is part of a reply to an angry sender who thinks you didn't read the email or read it wrong, in which case they will definitely be listening.
I've tried. Most people are using tools to construct the emails, and some have absolutely no idea that there are text and html components. None. Nada. Zip. They are completely ignorant of these issues.
In part that's because the tools they use deliberately and explicitly hide the details from them.
The same is true of browsers and email clients. The URL bar is disappearing from browsers, and when was the last time you tried an in-line reply to an email using GMail? It's hard. Really hard. The providers of interfaces are working to hide these details from you.
So "Tell the sender" is, in my direct personal experience, pretty pointless. I still try, though:
"God grant me the courage not to give up what I think is right even though I think it's hopeless. -- Chester W. Nimitz"
Tell the sender. If no one does, they won't know of this problem.
Of course, sometimes it happens that telling the sender is part of a reply to an angry sender who thinks you didn't read the email or read it wrong, in which case they will definitely be listening.