C: C1 CAPABILITY
  S: * CAPABILITY IMAP4rev1 PREVIEW SEARCHRES
  S: C1 OK Capability command completed.
  [...a mailbox is SELECTed...]
  C: C2 SEARCH RETURN (SAVE) FROM "FOO"
  C: C3 FETCH $ (UID PREVIEW (LAZY))
  S: C2 OK SEARCH completed.
  S: * 5 FETCH (UID 13 PREVIEW "Preview!")
  S: * 9 FETCH (UID 23 PREVIEW NIL)
  S: C3 OK FETCH completed.
  [...Retrieve message 9 preview in background...]
  C: C4 UID FETCH 23 (PREVIEW)
  S: * 9 FETCH (UID 23 PREVIEW "Another preview!")
  S: C4 OK FETCH completed.