In an interview with Reason Magazine, Bruce Schneier says:

I remember being told as a child: "Never talk to strangers." That's actually stupid advice. If a child is lost or scared or alone, the smartest thing he can do is find a kindly looking stranger to talk to. The real advice is: "Don't answer strangers who talk to you first." The difference is important. In the first case, the child selects the stranger-and the odds of him selecting a bad person are pretty negligible. In the second case, the stranger selects the child; that's more dangerous.

This is exactly why unsolicited email messages (or phone calls) should be treated with suspicion.