The dreaded 0x800CCC0F sending error

Error 0x800CCC0F,is typically encountered when there is a credential issue with the account - very frequently with Yahoo / AOL / AT&T accounts where the user has not used an <App Password> as the email address for the account in Outlook.

You've gone though everything I can think of from a trouble-shooting standpoint so the only thing left is making sure that Windows is completely up-to-date.

This may also seem like an odd-question to ask, but you didn't turn off 2FA on the Google account after you generated the <App Password> did you? (reason for asking is that one user who was encountering issues did so - just covering all bases)

Outlook MVO Diane Poremsky (the author of the article you referenced) has a variety of articles / reference points on the subject (she is the pre-eminent expert in all things Outlook) which brings the focus back to the <timeout> setting for the account

Error 0x800CCC0F when sending or receiving email.

Usually caused by Antivirus software scanning email, but antispam scanners may cause Outlook to return this error as well. Disable the email scanning or increase the timeout setting. Also, make sure your firewall is allowing the scanner software to connect to the Internet.

as described in

Common Outlook Outgoing Server (SMTP) Errors

https://www.slipstick.com/problems/common-outlo...

Outlook 2016 0x800CCC0F 'The connection to the server was interrupted'

https://forums.slipstick.com/threads/95290-outl...

Windows *was* completely up to date, when the error first appeared; in fact, I had correlated the problem with the three July updates to Windows 8.1. I backed out those updates, but no change; (*sigh*). They're ready to get applied again when I reboot.

I left 2SV on after I set an app password, to no avail. But Diane's article says my version of Outlook will work fine w/o 2SV and an App PW, so I've since backed them out for both my GWA account and my GMail account.

I only run Symantec Endpoint, which I temporarily disabled as a test, again, to no avail. I don't have any other AV (e.g., Defender) enabled. The firewall is obviously not implicated, since test messages are sent/returned to my inbox.

I'll review those other articles, thanks.

Really wish you had another computer to test this on - the only elephant in the room is Win 8.1 - not that I know of any reason why that should impact anything

As an aside, what is timeout setting for these accounts?

<CTRL+ALT+S> --> <Edit> ==> <Select Account> --> <Account Properties> --> <Advanced Tab>

If you increase the time - does it help any?

It appears there's another elephant in the room: I configured Mozilla Thunderbird to talk to my GWA. Like Outlook, mail gets delivered to my inbox. Also like Outlook, outbound sends fail:

I've increased the timeout from the default 1 minute to 3 minutes in Outlook, but still no love:

Task 'Dan's '59 Mail - Sending' reported error (0x800CCC0F) : 'The connection to the server was interrupted. If this problem continues, contact your server administrator or Internet service provider (ISP).'

So the common denominator would appear to be Google, since both my GWA and GMail accounts are failing, and both are hosted by Google. I also have a Yahoo account, but attempts to configure Outlook to talk to it via IMAP fail (with or without 2SV/App PW). I was hoping for a differential diagnosis using an account not hosted by Google, but I'm striking out.