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

I hope Avast has some redeeming value. I often think of what would happen if I am able to uninstall Avast. One thing (aside from this particular issue with Outlook) bothers me is that almost every time I connect to my bank I get a message from Avast "everybody sees what you are doing" attempting to sell me some other add-on.
I will read the article you referenced. Thank you.