C: a001 CAPABILITY
   S: * CAPABILITY IMAP4rev2 STARTTLS LOGINDISABLED
   S: a001 OK CAPABILITY completed
   C: a002 STARTTLS
   S: a002 OK Begin TLS negotiation now
   <TLS negotiation, further commands are under TLS layer>
   C: a003 CAPABILITY
   S: * CAPABILITY IMAP4rev2 AUTH=PLAIN
   S: a003 OK CAPABILITY completed
   C: a004 AUTHENTICATE PLAIN dGVzdAB0ZXN0AHRlc3Q=
   S: a004 OK Success (tls protection)