>>>Thank you (and others who contributed to this thread) very much for your helpful input. But here is a twist. I have not done anything related to this problem yesterday. I just turned on my PC (it is a holiday in the USA today so I don't have to be in the office early); then started Outlook and all emails - including GMAIL - works. Another strange thing is that today, when I loaded Outlook, I didn't get the message "the server you are connected is using a security certificate that cannot be verified. Do you want to continue using this server" (the image 1 I attached to my earlier message to you). Which leads me to believe that the problem does have something to do with the security certificate. For now I will document all that I experienced with this problem because you never know if/when it happens again.
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>>Perhaps you weren't the only one with the problem, so some noise was raised elsewhere and thus became the squeaky wheel which gets oiled on a weekend? IOW, something was updated somewhere beyond your network cable, and the pieces now talk to each other much better than before?
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>It is possible. The concern I have is that if/when it happens again I don't know if I will need to shake my computer, restarted it, turn the light in the office on and off, or post more UT messages, to get this resolved.

I agree with Dragan, most likely Avast slipstreamed in an update which addresses your issue.

Don't you just love how the Avast product you paid for is making your life easier?

http://robert.ocallahan.org/2017/01/disable-your-antivirus-software-except.html