I have the same exact setup working here. Mac mini running VPN, HTTP, and Email. It's behind an Airport Extreme running as a router however.
When I telnet to my port 1723 from outside, I get a connect and the server waits for input. When I do the same to yours, I get a connect and then an immediate disconnect.
said by "Your System" :[bob@vs1476 ~]$ telnet server.djswirkmke.com 1723
Trying 99.139.169.210...
Connected to server.djswirkmke.com.
Escape character is '^]'.
Connection closed by foreign host.
said by "My System" :[bob@vs1476 ~]$ telnet che.blythe.org 1723
Trying 142.54.105.162...
Connected to che.blythe.org.
Escape character is '^]'.
My firewall has TCP port 1723 forwarded to the server and UDP ports 500, 1701, 4500 forwarded to the Server as Per Apple's setup instructions.
On your system there is definitely something answering up on TCP port 1723, are you sure it's the server?
Also, what client are you attempting to connect with? I have been using iDevices and Macs configured in Network Preferences. I also have the server setup to use L2PT with a shared secret, but have also used PPTP without a problem.
There is definitely a difference in the way our 2 systems respond on port 1723 however. Says perhaps the trouble is with the server or routing to it.
EDIT: Oh, one more thing I remember. I had to disable "Back to my Mac" because there was a port conflict. Don't remember exactly, but I did have to turn it off. I remember thinking no big deal because VPNing into my private network was as good or better than Back to my Mac.