I can imagine that some race condition slips into production, but what's hard for me to process is why it’s 5 or 6 years later and it’s still not fully fixed.

I mean, even if you have no idea what's the cause, you e.g. stuff counters everywhere and when they don't match you send the telemetry with the details. Privacy is preserved and over time you get an idea what to look for.

I admit I have no idea how mail client works, but clearly there must be some way they could pinpoint it, with such large userbase..