PPTP port forwarding

I have a Debian server (with public IP) and Raspberry-PI at home with ADSL connection. I am running a web server on RP which I want to be accessible from the outside.

I have set up PPTP and my RP connects to my server. The IP of RP is 10.0.0.100; the IP of gateway is 10.0.0.1.

I have set up the port-forwarding with IP tables on server like this: (xx.xx.xx.xx is my public ip).

iptables -t nat -A PREROUTING -d xx.xx.xx.xx -p tcp --dport 80 -j DNAT --to-dest 10.0.0.100:80
iptables -t nat -A POSTROUTING -d 10.0.0.100 -p tcp --dport 80 -j SNAT --to-source 10.0.0.1

I have also set net.ipv4.ip_forward = 1

The port 80 is open both on server and on RP.

But because of one strange reason I cannot connect to RP.

When I do curl from server like this: curl -I http://ift.tt/14sf8Nu I got positive response. So connection between server and RP is working. But when I do curl form outside like this: curl -I http://xx.xx.xx.xx/ I got curl: (7) Failed to connect to xx.xx.xx.xx port 80: Connection refused

What I am doing wrong?

Routing table on RP is like this:

Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
10.0.0.1        0.0.0.0         255.255.255.255 UH    0      0        0 ppp0

and ifconfig of RP shows this:

ppp0      Link encap:Point-to-Point Protocol  
          inet addr:10.0.0.100  P-t-P:10.0.0.1  Mask:255.255.255.255
          UP POINTOPOINT RUNNING NOARP MULTICAST  MTU:1496  Metric:1
          RX packets:16 errors:0 dropped:0 overruns:0 frame:0
          TX packets:14 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:3 
          RX bytes:678 (678.0 B)  TX bytes:7058 (6.8 KiB)