Error 721 is a Microsoft error which is encountered after making an unsuccessful attempt to establish a virtual private network (VPN) connection to a corporate network by using a Point-to-Point Tunneling Protocol (PPTP) client. You may even get an error 721 after making an unsuccessful attempt to establish a dial-up connection after upgrading your computer to Microsoft Windows XP from earlier versions. The exact error 721 description may vary from 'Remote PPP peer is not responding' to 'the remote computer is not responding'.
Cause of Error 721
Point-to-Point Tunneling Protocol uses Generic Routing Encapsulation (GRE) for tunneled data and the error may occur if the network firewall does not permit the GRE protocol traffic. The Generic Routing Encapsulation protocol is used along with Point-to-Point Tunneling Protocol to create virtual private networks between the server and the client or between the clients. You may even encounter this error if the modem driver is not compatible with Windows XP.
How to fix the Error 721
To resolve this error, you need to configure the network firewall so as to permit the GRE protocol 47. Also, you need to make sure that the firewall permits the TCP traffic on port 1723. Both of these conditions are required in order to establish the virtual private network connectivity by using Point-to-Point Tunneling protocol.
Installing the latest Windows XP compatible driver for the modem can also fix the problem in some situations.