Just found and fixed the problem, as it appeared on my machine – Win 10 PRO, Office/Outlook 2013 – recognize that at least one not-universal configuration issue likely means that this has limited transferability, but it solved the problem, quickly and easily, for such a setup.
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My cable modem provides two WiFi SSIDs, but [for most purposes] resolves them to a single/common "network" for communications: my printer, for ex, requires that I connect via WiFi, in order to allow direct printing from handheld/mobile devices, BUT, tho I connect the printer to my "network" using TXXXXXG SSID, my desktop computer is connected to TXXXXXG-5G SSID, yet they play nicely together.
I've run this printer for several months, with no problem, but recently, seemingly out of the blue (poss related to recent Windows Updates), I started getting the Outlook "Metered Connection" on startup error. I checked, and everything everywhere seemed to be set correctly, including the Metered Connection switched "Off" in Network Config. To test, I re-enabled the ethernet connection, and no problem (on starting Outlook). Then, I disabled that, and connected to TXXXXXG SSID, and again, NO PROBLEM! Then, I connected to the TXXXXXG-5G SSID [NOTE: I closed and restarted Outlook after every networking change] and got the "Metered Connection" error, again.
THIS TIME, instead of going to Network & Sharing Center, or other Setup interfaces, I directly clicked the wireless icon in my systray, LEFT-clicked on the TXXXXXG-5G SSID listing, then Properties, AND…
For whatever reason, IN THIS INTERFACE ONLY, the "Metered Connection" option – for this sub-SSID (the -5G version) – was checked/Set as "On" – I turned it off, restarted Outlook, and, suddenly everything is working again.
Hope this helps someone. All the best,