title: golang "x.509 unknown authority" on smtp, but no error on tls
body: <p>When trying to send a mail with golang to my local postfix server I get the error<br>
  x509: certificate signed by unknown authority<br>
  Consider the following code:</p>
  
  <pre><code>config := &amp;tls.Config{ServerName: "example.com"}
  _, err := tls.Dial("tcp", "example.com:443", config)
  //err == nil
  c,_ := smtp.Dial("example.com:25")
  err = c.StartTLS(config)
  //err == x509: certificate signed by unknown authority
  </code></pre>
  
  <p>Apache and postfix both use the same cert signed by StartSSL.</p>
  
  <p>How comes it is accepted in one instance and rejected in the other?<br>
  As a sidenote: is it possible to get more information on the certificate in the smtp case?</p>