HTML e-mail is the use of a subset of HTML to provide formatting and semantic markups capabilities in e-mail that are not available with plain text.
Most graphical e-mail clients support HTML e-mail, and many default to it incorporating both a GUI editor for composing HTML e-mails and a rendering engine for displaying received HTML e-mails.
HTML mail allows the sender to properly express quotations (as in inline replying), headings, bulleted lists, subscripts and superscripts, emphasized text, and other visual and typographic cues to improve the aesthetics and readability of the message, as well as semantic information encoded within the message, such as the original author and Message-ID of a quote. Long URLs can be linked to without being broken into multiple pieces, and text is wrapped to fit the width of the user agent’s viewport, instead of uniformly breaking each line at 78 characters as was necessary on older text terminals. It allows in-line inclusion of tables, as well as diagrams or mathematical formulae as images, which are otherwise difficult to convey.
Many mail programs these days incorporate Web browser functionality resulting in the display of HTML, URLs, and images. This easily exposes the user to offensive images in spam. Additionally, spam written in HTML contains web bugs that allow spammers to see that the email address is valid and their message has not been caught in spam filters. JavaScript programs make it difficult for the user to close the advertised page or direct the user to another web page. Spam messages take advantage of these security lapses, using these holes to install spyware. Mail clients, which do not enable HTML in their messages, are pretty much safe and have fewer risks when compared to those who have these functionalities enabled.
