C: B1 FETCH 1:4 (ENVELOPE PREVIEW (LAZY))
  S: * 1 FETCH (ENVELOPE ("Wed, 23 Sep 2020 15:03:11 +0000" [...])
     PREVIEW "Preview text for message 1.")
  S: * 2 FETCH (PREVIEW "" ENVELOPE
     ("Thu, 24 Sep 2020 12:17:23 +0000" [...]))
  S: * 3 FETCH (ENVELOPE ("Fri, 25 Sep 2020 09:13:45 +0000" [...])
     PREVIEW NIL)
  S: * 4 FETCH (ENVELOPE ("Sat, 26 Sep 2020 07:11:18 +0000" [...])
     PREVIEW NIL)
  S: B1 OK FETCH completed.
  [...Client has preview for message 1 and knows that message 2 has
      a preview that is empty; only need to request preview of
      messages 3 & 4 (e.g., in background)...]
  C: B2 FETCH 3:4 (PREVIEW)
  S: * 3 FETCH (PREVIEW {30}
  S: Message data from message 3.
  S: )
  S: * 4 FETCH (PREVIEW "Message 4 preview")
  S: B2 OK Fetch completed.