start_tls_session

# File lib/net/imap.rb, line 1512
    def start_tls_session(params = {})
      unless defined?(OpenSSL::SSL)
        raise "SSL extension not installed"
      end
      if @sock.kind_of?(OpenSSL::SSL::SSLSocket)
        raise RuntimeError, "already using SSL"
      end
      begin
        params = params.to_hash
      rescue NoMethodError
        params = {}
      end
      context = SSLContext.new
      context.set_params(params)
      if defined?(VerifyCallbackProc)
        context.verify_callback = VerifyCallbackProc
      end
      @sock = SSLSocket.new(@sock, context)
      @sock.sync_close = true
      ssl_socket_connect(@sock, @open_timeout)
      if context.verify_mode != VERIFY_NONE
        @sock.post_connection_check(@host)
      end
    end