When you receive an email in your inbox, there is usually a small icon next to it that you can click on to mark the message as unread. This can come in handy if you want to make sure you don't forget to read an email later. However, sometimes the icon is hidden or you accidentally mark the message as read when you didn't mean to. If this happens, you can still mark the message as unread by right-clicking on it and selecting the Mark as Unread option from the pop-up menu.