Are you building HTML Newsletter? Following are some basic things you need to follow up before sending HTML email:

• Remove HTML tags like DOCTYPE, html, meta tag, head, link, title and Comments from your mail content.
• Because of limited support of stylesheets in webmail client, Use of table is best to layout html email
• Hide Image Toolbar
• Dont Use Anchors
• Use Inline Stylesheet
• Create your design with a white background color and black font color
• Use
  instead of <br /> instead of <p>
• Strip out forms and Javascript
• Provide a URL for display in the browser
• Send your e-mail as multipart/alternative