Error 721: Remote PPP peer or computer is not responding.
Resolution: If you have tried many thing other people suggest like rebooting, reloading hardware and re-installing the VPN or dial in connection, you still get the same problem. I will suggest to check the router settings and make sure TCP Port 1723, IP Protocol 47 (GRE) are opened. Also make sure that the router has the PPTP enabled and not firewall block the traffic. On the RAS server, check the DHCP settings.