title: Why does email need an envelope, and what does the "envelope" mean?
body: <p>What is the exact difference between the envelope and the email in smtp?</p>
  
  <p>Why does the protocol need the envelope? In snail mail, the letter needs not contain addresses and is not visible to the postman (at least that's what you want to believe), so all the routing is made entirely based on the envelope and that is clear to me.</p>
  
  <p>However in e-mail, the letter itself is parseable and has headers such as From: and To:. Why is that not enough to route email? Why do we need an envelope?</p>
  
  <p>And what is the meaning of a divergence between the envelope and the email itself? Does it necessarily mean that someone is trying to game the system, or is it, under certain circumstances (what circumstances?), legal to have a difference between them.</p>
  
  <p>Your clarifications are more than welcome.</p>
  
  <p>Puzzled email receiver.</p>