In 1971, Ray Tomlinson was experi- menting with ways people and computers could interact. When he sent a message from his computer through a network to a different unit in his office, he had sent the first e-mail. Now decades later, more than a billion e-mails are sent every day. Many contain important news from family and friends, but others may carry unwanted advertising or a destructive virus. A basic rule governing e-mail use is: “Don’t open it unless you trust the sender.”