电子邮件中的动态HTML内容

I have seen some mails which has HTML content embedded in them. The content of the mail changes as the corresponding webpage in their site change(for example price of stocks which keeps updating in the mail itself). How to link such webpage content into emails? In other words how to link a web page into an email so that whenever I change the html page, the mail content also changes. Thanks...I have seen some mails which has HTML content e