Plain text does not support formatting such as bold, italic and color fonts. The format cannot carry pictures or graphics in the body text, although a user can include such objects as attachments in an email. HTML, on the other hand, creates content that’s similar to traditional documents and supports various fonts, colors and bullet lists. A recipient of an HTML message sees it as it was sent.
Plain text takes up less bandwidth and may not be flagged as spam. Since HTML sends richly formatted messages, it’s the best choice only when the receiving program can interpret it; otherwise its intended recipient might be unable to retrieve it.
An HTML email can easily be made to look like the professional format that an individual may receive from an authentic organization, such as the bank. Since the mail format is more susceptible to malicious use than plain text, a user is advised to open it only if it comes from a credible source. For instance, an HTML mail may carry a bug that tells the sender if the receiver has opened it and this may expose the receiver to more spamming.
A user can configure his email application to send messages in the preferred format.