HTML Email is a type of email message formatted with the HTML markup language. This means that instead of writing plain text, the authors of email messages formatted in HTML can change the font size and colour of their text, and can add pictures, tables, and other objects to the mail message. However, there are many compatibility problems with HTML Email, as well as security and privacy issues. For these reasons and others, many people oppose the use of HTML in personal and business email messages. On many popular mailing lists, the use of HTML mail is grounds for removal from the list, and many veteran list members ignore all email formatted with HTML.
The use of HTML to mark up an email message does not guarantee that the receiver of the email message will see the text as the author intended. This is because different email clients render HTML differently, much how different web browsers render HTML differently. However, the differences in email client rendering are often much more significant than the differences in browser rendering, as HTML rendering is not a core function of an email client. Support for HTML attributes such as CSS are often incorrect or lacking altogether. Also, fonts specified that are not installed on the readers computer will be displayed in a default font, negating any effort on the part of the sender to improve the appearance of the message. Additionally, HTML email is often two to three times the file size of a text email, even if the message is not formatted at all. This is because in addition to the message text and the formatting, the message text is often repeated in plain text so that non-HTML compliant email clients will have something to render.
HTML email is also considered one of the driving forces behind the spam marketing industry. HTML allows a website link in an email message to go to a different place than the text states. This is often exploiting in "phishing" attacks where users are lured by an email forged to appear as though it came from a legitimate company (such as a bank), and used to capture sensitive information. Also, HTML email can contain references to images on the web instead of embedding the images in the message itself. Thus, when a message is read, the server hosting the image in question is made aware of the fact that the message is being read. Thus, spammers and other malicious entities can track users and verify their presence. For modem users, the call to an image on the web can cause the computer to automatically connect to the internet, costing money. An additional drawback to the use of HTML is that mail formatted in HTML takes much longer to render (to appear on the screen) than a regular text email. This is because the email client must parse the mail and create a rendered page, much like a webpage is rendered. For slow or overloaded computers, the performance hit can be significant.
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