Why does the server answer "D"?

Question

C++ / C#

I'm trying to connect to a server using SSL. On a regular text connection, the server responds adequately, and after the starttls command it writes 220 go ahead and ignores the packets. It receives packets sent by the send_to_server_ssl function and responds to them with a mysterious "D" symbol (with CP_WINANSI disabled - "-"). Why does he respond like that and how can I make friends with him?

void send_to_server_ssl(mbedtls_ssl_context s,const unsigned char* text){
  int size=1024;
  unsigned char* req=new unsigned char[1024];
  mbedtls_ssl_write (&s, text, strlen((const char *)text));
  mbedtls_ssl_read(&s, req,size); 
  cout << "@:";
  for(int i=0; req[i] && i<size;i++)
    cout << (char)req[i];

  delete [] req;
}