E-mail is a standard means of communication on the Internet. As a way to send messages to anyone in the world, instantly and for free, it became one of the first killer features of the Internet and a harbinger of many things to come.

E-mail is an old technology, designed in the early, naive days of the Internet. Initially, every mail server accepted all messages from anyone and forwarded them on to any destination. E-mail accounts were protected by passwords, but those were symbolic—sent as plain text over unencrypted connections, and the word, "spam" was still associated mainly with the Monty Python sketches.

Unfortunately, as more and more people came online, many malicious users started to abuse the e-mail system. E-mail became a free way to send marketing information with the ability to reach every person on the planet. This caused an explosion of unwanted e-mail messages, most commonly called spam, which, at its peak, made up more than 95 percent of e-mail traffic. These...