S: * OK [CAPABILITY IMAP4rev2 STARTTLS AUTH=GSSAPI
   LOGINDISABLED] Server ready
  C: A01 STARTTLS
  S: A01 OK STARTTLS completed
  <TLS negotiation, further commands are under TLS layer>
  C: A02 CAPABILITY
  S: * CAPABILITY IMAP4rev2 AUTH=GSSAPI AUTH=PLAIN
  S: A02 OK CAPABILITY completed
  C: A03 AUTHENTICATE PLAIN dGVzdAB0ZXN0AHRlc3Q=
  S: A03 OK Success (tls protection)