If you long press [ Deleted ] folder for this account in the account list, the first item in the popup menu will show what server folder it syncs with (it's disabled, so it won't be selected, but should still be visible).
If it says anything but Trash (which is Yahoo's server side "trash" folder), please remove the account in my app and add it back.
( screenshot shows my Yahoo account, which I keep around for tests... )
