I had already touched on this subject and intended to expand on it here... but Carlos Diaz somehow took the wind out of my sails, so I invite you to read his article on the subject.
So there's no need to go back over what's been written elsewhere, but I do have a few things to add.
Why has email become the business killer app?
Because, on the face of it, everyone knows how to use it. You know how to write, you have someone's address, so you know how to send...