How do you delete messages? If you open them then they are technically "read". If you use right-click and delete from the context menu, then they haven't been read and so would still be, in theory, unread. I do see many unread messages in both Junk and Trash, but they are put there by junk controls or filters, so are untouched by human eyes. But this confirms that simply being put into Trash doesn't automatically mark them as read.