... client connects to server and negotiates a TLS
      protection layer ...
  C: C01 CAPABILITY
  S: * CAPABILITY IMAP4rev2 AUTH=PLAIN
  S: C01 OK Completed
  C: A01 AUTHENTICATE PLAIN
  S: +
  C: dGVzdAB0ZXN0AHRlc3Q=
  S: A01 OK Success (tls protection)