Difference between Plain Text and HTML Emails
Email was invented as a way of being able to send text messages internationally, regardless of the programs being used by those that were sending and those that were receiving.
As a result they were created as plain text (text with no styles - such as Bold, Italics on Underline).
An HTML message isn't plain text - it enables you to add pictures, colors, etc to your messages. It can only be viewed properly (like a web site page) if it is opened in email software that can view it (older versions of Outlook or Outlook Express for example, will receive the picture elements as an attached file because they can only “see” the text element).