Question : Problem: error 721 using a VPN PPTP connection

When logging into my domain at work with a SBS2003 it says "verifiying username and password" then gives error 721. ports TCP 1723 and IP 47 are directed at the server with my Linksys router that supports PPTP passthrough, my account is allowed full access under the Dial-In tab. When VPN was set up on server I chose Custom setup because of only one Ehternet connection is on the server. The DHCP scheme is the same as the LAN addressing and is not in the range of the other workstations.

Answer : Problem: error 721 using a VPN PPTP connection

Error 721 usually means the client cannot see tcp 1723 on the VPN server.  

Here's a link telling you how to setup RRAS on a single-NIC machine:

http://www.pctechnicians.ca/help/singlenic.html