I guess I should have requested help with a workaround to this issue. I know it is not specific to Apple Mail.
In my case, It is most assuredly Frontiers servers having issues. This is an ongoing issue with Frontier. It has been happening for years.
Thanks for your help and suggestions Drew.
This issue occurs with other mail applications as well. MS Outlook Mail does the same thing.
This is a timeout issue with Apple Mail (the mail app that comes with OS X) and MS Outlook Mail as I searched Apple support groups and other people have the same issue.
I requested Apple add a feature to allow changing Mail's timeout setting to something other than the default.
Frontier support is useless. They walk you thru prewritten instructions and when they exhaust all the steps to try they say they can't help. When I contacted their upper tier support they said their servers get busy at times and I would have to live with the issue.
I switched to Spectrum, which is much more reliable and faster Internet access, but they have nothing to do with Frontier mail issues.
I have exhausted all the Apple Mail's troubleshooting tools with no change in behavior.