Windows 10: Lost "more" folders content

You have to create the extra Folders in the Win 10 Mail App. Then that same folder will be synced with the corresponding Yahoo folder(Webmail interface).

In the Win 10 Mail App use the Folders > All Folders + (create new folder).

There is no "more" folder in my Yahoo Account.

 

"I have found that I can go back to my Yahoo account and the content of the "More" folders is still there. Just don't understand why it's not there in the new mail app."

Refer to my attached screenshot below. I have Yahoo acct synced in Mail app. Click 'More' at bottom of menu. You should then see all your folders in your account in a new column on R. In my case, I created one called 'Archive' (same purpose as you, to store keys and purchases for warranties). If you r-click any folder, there are options to move it to the main 'Folders' column on L or not sync, say the Junk folder so spam is not put on your PC. As pointed out prior, create the folder with the same name as the one in your acct, if it won't auto appear. Also, review your sync settings in Settings/Manage Accounts/Yahoo/Change mailbox sync settings/Download email from and ensure this is set to 'any time',as opposed to1-3 months, to get all your online content synced, as you see it online with all the ads. Good Luck