The vast majority of HTML emails do not comply with World Wide Web Consortium (W3C) HTML standards. This can cause rendering and delivery issues, particularly at AOL, MSN and Hotmail. AOL, for example, has a that is an HTML validator, which scans incoming messages for HTML syntax and formatting errors. If it detects invalid HTML, it will reject the message.
If you use HTML in your messages, make sure your code is error-free and follows W3C HTML standards. Popular HTML-editing software such as Homesite or Macromedia Dreamweaver already offer effective validation tools and will highlight any errors as you create your message. For a complete reference specification of HTML formatting, visit the World Wide Web Consortium documentation pages. Also, you can use the HTML validator in your email application or a third-party validator such as W3C Markup Validation Service.