def enable_starttls(context = SMTP.default_ssl_context)
      raise 'openssl library not installed' unless defined?(OpenSSL)
      raise ArgumentError, "SMTPS and STARTTLS is exclusive" if @tls
      @starttls = :always
      @ssl_context = context
    end
Enables SMTP/TLS (STARTTLS) for this object.
+context+ is a OpenSSL::SSL::SSLContext object.