If you try to establish a virtual private network (VPN) connection to an enterprise network using a PPTP (point to point tunneling protocol) client, the connection to the Microsoft Windows Server remote access server may fail. You can receive the following error message:
Error 721: The remote computer is not responding.
Note that the description of error 721 may vary.
This problem can occur if the network firewall does not allow GRE (Generic Routing Encapsulation) protocol traffic. GRE is the IP 47 protocol, PPTP uses GRE for tunnelized data.