Thank you. Tried that multiple times (doing the same thing and expecting different results).
Thank you. This is the first time I have heard anyone suggest that Mail filters caused the problem that a small percentage of users had with Mail after updating macOS, and I suspect that I failed to communicate the root issue. (My search skills are not good, but Is It Safe to Upgrade to macOS 11 Big Sur? mentions this, if you want to read more.) As far as I could tell, the cited Apple support page was just general information and did not address the recurring problem. Nonetheless, I plan to verify that filters are off on the problematic iMac tomorrow.