HTML is basically a web page that appears when you open your e-mail. This might be fine when you are signed up with Netflix or Neopets, but it is not fine when you open up a spam advertisement.

Spammers use HTML to plant viruses that harvest your address book, or tell them when you have opened one of their e-mails. In most e-mail accounts there are settings to disable the HTML that would normally be displayed in sent emails. It is important to disable this feature because your computer and e-mail will be at less risk of receiving the viruses implanted by spammers. The opened email will not automatically download and display images or attachments.