It might not hurt to print out the responses above and read them slowly and carefully, maybe over a cup of coffee/tea, mainly away from the computer.
The biggest problem with forums help is that the suggestions tend to be in no particular order and sometimes not even in any order of importance. It is tempting to just start trying things without any order or reason. I do it all the time!
Strangely, "frustration" seldom leads to better or quicker solutions.
Basically, do the easiest things first, realizing that they may not work but that there will be more complicated steps to follow. But even the seemingly useless steps may solve problems you don't even know about. And those combined 'minor' problems may even be the actual problem. The hardest part of problem solving is taking
each step, one at a time.
You
can do it!
It appears to be working fine now - I rebuilt my mail folder.
Thank you all.