A love letter or sensitive memo goes to the wrong party. You and many other e-mail users then buy or sell the stock. The huckster cleans up. Messages sent from work are owned by your employer, who might monitor them. System operators on many on-line services also can monitor messages.
Packet sniffers are illegal to all but law enforcement agencies.
Many hackers have them, however. Four-letter words, for instance, could trigger an alert. It is rare in the big universe of e-mail, specialists say, but some companies experience it frequently.