Apple finally uploaded the 10.15.3 Combo Updater. I have downloaded and installed it. The install took quite a while. Just on principle, I did a Restart after everything was booted up. All seems to be well. I cannot test the Mail App to see if the Email loss problem was fixed, but it appears to be. For your information this article lists all the processes that have been changed in the OS update. There are a bunch of them. Everything is still very speedy on my iMac. It seems to me my iMac Retina display has a little richer color and is a bit brighter, but that is purely subjective.