why won't an email load images

Now at least I have work-arounds if this happens again. I think your diagnosis of corruption is correct. When I search "how do emails become corrupted" I get

"Causes for Email Corruption

1. Viruses or other Malicious Software:- In computer system, many viruses can infect and damage the emails and makes them inaccessible.
2. Terminating System Abnormally:- If system/application is quit abnormally, then emails are prone to get corrupted or damaged."

I vaguely recall Mail hanging around that time so a bug seems most likely to me. Thanks for your time.

No that doesn't seem to be it. I tried your suggestion—more than once but to no effect. "Load remote content in message" was ticked off but I tried turning it on and off. I've archived that email so that if I see possible solutions I can check them out. On the other hand the problem has not repeated with other emails. The problem email still won't load remote content whether or not "Load remote content in message" is ticked off. Puzzling.

Thanks that seems to be the explanation or diagnosis. I forwarded the offending email from my new iMac, where it won't load, and it still wouldn't load but then I forwarded it from my older iMac and then it would load on my new iMac. This was an email sent to an iCloud address and I was also able to open the original at iCloud.com on the new iMac.

So how does that happen? The computer has a wired connection so not a Wifi issue.