Apple Feedback I
I (currently) use Apple Macintosh at work.  Work uses Exchange.  I also used Apple Mail with several email accounts including my Exchange-based work email.  This was all working fine until Catalina.

Mail usually properly downloads mail messages (and events) from the Exchange server.  As I use Apple Calendar, I can accept events either from the email, itself, or via the Calendar program.

However, the status change of an email (that it is read, deleted, etc.) is not properly propagating back to Exchange.  I see this because OWA and Outlook are not getting updates.  As I read a message in Apple Mail, it still shows up in Outlook as unread.  If I delete a message in Apple Mail, it still shows up (and as unread) in Outlook.

I have Appleâ  s iOS Mail clients (iPad and iPhone) configured for the Exchange server, as well.  If I do any of these operations from from either iOS devices, it all works as expected; a read email message shows up as read in the Outlook client; a deleted email is deleted.

Additional feedback/update:
I ran Microsoftâ  s MacOS Outlook client but did operations within it.  When I read or deleted an email message there, the Apple Mail clientâ  s message quickly reflected the changes; when I deleted an email in the Outlook client, without touch the Apple Mail client, it disappeared from the list.  Same with reading one on the Outlook client; the corresponding mail message in Apple Mail changed to unread (the bullet disappeared and the font change to non-bold).


Apple Feedback II
I have Apple Mail configured to support access to (and downloading of) messages from different types of servers for my email accounts, including my work Exchange account.

As I am going through the list of incoming email messages in my Exchange account, and I come across a series of two or more consecutive emails that have the same subject, as I scroll down (with the arrow key) to the next message, or even select the next message by single-clicking on it, in a series with the same subject content, only the contents (and headers) from the first message will display.  I can only see the actual message by double-clicking on subsequent messages.

See the video.  In the video, I have Apple Mail (with custom toolbar arrangements), I first I am voting to an email message from Sam sent at 2:56PM.  I then select via single clicks the next three messages (Andrew, Paul, and again Andrew).  These have the same subject text.  Not that only the contents (and headers) of the first one is displayed.  I then go back to Samâ  s message but use the down arrow to move to subsequent messages.  Again, only the first one is displayed.

I then double click on a subsequent message to show that it is, indeed a different message, showing properly.

This happens in the reverse direction as well.   I start at Benjaminâ  s message and scroll up and then double click on the top message to show it is different.


Apple Feedback III
I have several POP accounts, in addition to an Exchange, Yahoo, and IMAP accounts.

Up until Catalina, I had â  Check for new messagesâ   set to 1 minute.  Several of my POP accounts servers were bouncing me for checking too frequently.  I later set this to 5 minute checking intervals.  Still, I was getting bounced and error emails.

Finally, I sniffed the network and, indeed, the Apple Email client was was starting new email transactions (new logins) less than 15 seconds after the previous one finished.  Sometimes, it was shorter!  I did not check to see if the interval is between start of a truncation and the start of the next transaction, or between the end of a transaction and the beginning of the next; it should be the latter. 

I tried again with the latest Catalina beta and it is still the case.