I just discovered I was a life saver, thanks for the kudos everyone.🙂
Why? It's not your fault. It's because how Apple Mail stores mail, it simply is not robust enough.
A robust program assumes that it may crash at any moment while writing to any of its files, and that there will be undiscovered, rarely occurring bugs that will have similar effect. The damage it does needs to be detected later and then repaired automatically or degraded gracefully so only one mail is lost, and not all of them.
Evidently, Apple fails to do so or didn't do a thorough job.
For mail this is really urgent, because a mail program does many things simultaneously which increases the chance that things go wrong once in a while, and on top email comes from the wild world outside and can contain many types of junk that need to be contained. Mail server delivery data muck, deliberate attempts to hack your mailbox by mail, phishing attempts, html newsletters made by Neanderthalers or marketeers, other buggy mail programs, poorly made phising mails, regular spam, you name it.
So there are a lot of possible 'runaway cars' that may dent another in the parking lot, but this should never cause an entire parking lot building to tilt so all the cars roll to one side of the floor and while still there, can't open the doors anymore.
Computer science made many solutions for this the past 3 decades and it isn't even that hard to do, many file formats have auto-recovery features.
For example: You may have seen a movie on TV or internet, or skype conversation that goes into "weird blocks" a few seconds and then go back to normal. That's damaged data and auto-recovery of the data format kicking in to fix it or at least reduce the damage. Without it, you'd be looking at digital snow for the rest of the show. Apple does the latter to your mail.
In modern databases this is done all the time, because a company can't afford to lose all of its financial data because 1 record went bad or there once was a power outage. For a user, losing his/her mail can be just as catastrophic. If they can, Apple can. Come on.
But that answer won't help ya, unless someone at Apple actually READS these threads and feels intense SHAME that they did not listen to that inner voice saying: hmm it looks spiffy now but we really gotta work on the internals a bit more. No, data is not sexy. Yes, it's boring. But it's necessary. Remember Steve's step father taught his son that the backside of a drawer should also be made neatly. Dad was right folks.
Since this mail I'm going into my 3rd combat with Mocrosoft Exchange which is another factor that, while it should be better than regular mail, also is another rogue runaway car that dents the fragile internal file structure of Apple Mail.
Yours truly, an Apple fanboy.