SOLVED: Server error user is over quota, emails bouncing back to senders when mac/icloud storage isn't full

This was driving me crazy. People were telling me that their messages to my mac.com email were bouncing back saying things like: "Recipient inbox full" / "Your message couldn't be delivered to <address@mac.com>. Their inbox is full, or it's getting too much mail right now" etc. But our Family Plan showed less than 5% space used of 2TB, and I was archiving my mac.com emails off using Outlook for Windows, including Sent Items, keeping junk folders deleted/empty, and emptying deleted items every time I logged off. But when forwarding an item from myself to myself I got the message "Server error: '552 5.2.2 <address@mac.com>: user is over quota'" I knew I had to solve this.

After much struggling, we realized that Outlook for Windows was still keeping a copy of all Sent emails on the server. So while my Sent *Items* folder showed 2 items only, the Sent *Messages* folder of my icloud account had messages dating back to 2017. I logged in to icloud.com on the web, accessed the folder, selected all items, deleted, refreshed, then selected all items in the Deleted folder, deleted again, and refreshed a few times (it took a while, I think there were over 4,000 items in there) and the folder showed zero. Forwarded the problematic item to myself again, no problem, so it seems this was the fix required.