title: Authentication with STARTTLS and SSL/TLS
body: <p>As far as I understood, both encrypt the connection. However, SSL/TLS "forces" the mail client to encrypt the channel from the beginning. Now let's say I <code>telnet mail.server 25</code> and, after the ehlo, I am presented with STARTTLS as an authentication option that the server allows me (the mail client) to use. If I choose STARTTLS does it mean that I can authenticate myself (mail client) against the server (using my digital certificate)? why and how? </p>
  
  <p>I am doing as follows but I don't understand where does the client authentication part comes in:</p>
  
  <pre><code>telnet some.mail.server 25
  ehlo some.mail.server
  ...
  250-STARTTLS
  ...
  STARTTLS
  mail from: guy
  rcpt to: otherGuy
  data
  someting
  .
  quit
  </code></pre>
  
  <p>Shouldn't I be sending the client certificate to the server? I tried this using <code>openssl s_client -starttls smtp</code> which actually provides a -cert and -key options to specify the certificate and private key. </p>
  
  <p>So, in fact, the only disadvantage of SSL/TLS compared to STARTTLS is that since the connection is immediately encrypted, there is no way for the client to send its certificate on the same port. Only the server can send its certificate. Correct? A lot of confusion in my head... Please clarify this for me.</p>