Thanks for your reply and all the problem solving information RvW. I went to test it today and the mails are coming thru on Windows 10 mail today! I have printed your directions and will forward to Marcos the next time this issue comes up. (It's resolved itself before and then the problem started again a few weeks later).
Interestingly enough, ESET didn't load when I started my computer this AM and so the Windows 10 mail came thru. Then when I rebooted and ESET loaded, the WIndows 10 mail still worked.