I have been suffering, with all of you, the problems of Catalina Apple Mail and Exchange. I have included my reports to Apple via Feedback in the “additional text.”
I am writing today to report a potential solution.
On Thursday, I rejoined Apple's JointVenture support plan. I had purchased this previously but let its subscription lapse. For this single primary email problems reported here (my 1/28/2020 report in this thread), I rejoined at a cost of $500. I scheduled a service call for Friday afternoon. I was connected with a senior support person known to me only as Rebecca. She was knowledgable and did not talk down to me. It was actually quite pleasant. She proposed the solution described below.
As an aside, there was one additional problem I did not report: Apple Mail did not adhere to the "check for new messages" preferences setting. Regardless of whether I set to check the servers ever 1 minute or 5, once or twice an hour, Mail would connect to the server in much shorter intervals: as reported by one of my POP services, 1 connection every 0.1240 minutes (7.44 seconds)!! At about the same time, my service had updated their software so I wasn't sure who the offender was/. I finally sniffed the network and found that Mail was indeed coming in at sub-minute times between checking.
So, where am I going with all this?
All of the problems I experienced and reported in this thread have been fixed, including that one I did not report. At Rebecca's suggestion, I created a new user on my system and, once logged in, created a new Mail Exchange email account. I did the same for MS Outlook. As I had before, I ran them side by side. As I read an email in Apple Mail, within a few seconds, the email entry in Outlook changed from unread to read. When I deleted a message or moved it to another folder (on my system), the message disappeared from the list on Outlook. Outlook even reflected the flagging (though I didn't check for proper colors).
The rest of my 570k email messages spread in various mailboxes were exported from Mail in my old login and transferred to and imported to Apple Mail in my new account. As I keep all of my files (including photos and music) in a separate directory in /Users, the rest of the migration wasn't that painful. I avoided the migration utility or wholesale copying of stuff to avoid unnecessary and potentially troublesome state from coming over. I can detail more how I run my systems if anybody is interested.
My login on my Catalina has evolved through manual copying and eventually Migration Assistant from my earliest days on a Mac (I converted from UNIX to Macs in 1991). I could see older pref files and who knows what else. So, this maybe was a good thing to do - a little rebellion now and then is a good thing. As a side benefit, the fan on my Mac runs MUCH less than it had - subjective but noticeable.
I have been testing this solution as I enabled more and more of my environment (licenses, passwords, etc. for other applications) and little widgets (MenuMeters, JAC, Grabbit, FSMonitor, Microsoft Office products, etc.). So, for the most part, I'm pretty much up and running. But, the real test will come over the next several days during high activity on the Work Exchange server and me being tied to doing real work (MS Teams, etc.).
Other potential suggested fixes here have lasted a few days, if at all. So, if, by the end of this week, things are still working fine, I will declare victory and recommend more of you create a new account and start from fresh.
Sorry to be so verbose but this has been a significant pain for too long a time. My one regret is that there isn't enough information available in the system (or, seemingly, experience outside of Apple Engineering) to really debug and find the problem. But, if we are through this gauntlet, wonderful.
For what its worth, I will report back here near the end of the week (or sooner if the situation regresses).
aps
Configuration:
MacBook Pro (16-inch, 2019)
MacBookPro16,1
Memory: 16 GB 2667 MHz DDR4
Graphics: AMD Radeon Pro 5500M 4 GB
Intel UHD Graphics 630 1536 MB
Storage: 2 TB flash
Displays: Dual (side-by-side) 27" Thunderbolt Displays
Thunderbolt to USB-C adaptor
This system was purchased in March; the same was occurring on my previous MBP, a Mid-2017 MacBookPro14,3.