C: abcd CAPABILITY
  S: * CAPABILITY IMAP4rev2 STARTTLS AUTH=GSSAPI
   LOGINDISABLED
  S: abcd OK CAPABILITY completed
  C: efgh STARTTLS
  S: efgh OK STARTTLS completed
  <TLS negotiation, further commands are under TLS layer>
  C: ijkl CAPABILITY
  S: * CAPABILITY IMAP4rev2 AUTH=GSSAPI AUTH=PLAIN
  S: ijkl OK CAPABILITY completed