* A protocol that leaks metadata, including some message content, at the envelope layer.
  Could you elaborate more on this? I don't really understand what you mean.

He means:

  EHLO your.mx.example.tld
  MAIL FROM:<bob@example.tld>
  MAIL TO:<alice@recipient.tld>
  DATA
  From: Definitely Not Bob <bob@example.tld>
  To: Probably Not Alice <alice@recipient.tld>
  Date: Mon, 12 June 2017 16:02:43 -0500
  Subject: Super Secret EMAIL! Don't let Eve see!
  
  -----BEGIN PGP MESSAGE-----
  ...
  -----END PGP MESSAGE-----
  .
  QUIT

Thereby leaking at minimum the sender and recipient, as well as any intermediate hosts that relayed the email. Depending on how your MTA handles email, you can also leak subject and other metadata.