PPTP over Zyxel NAT and win2003 server

Hi,
I have a win2003 server behind a Zywal with full NAT. In the win2003
server I have 2 NIC's, each one with a privat IP and both public IP
over NAT. The DHCP server is on another machine, win2000 server, in the
same LAN. I setup a PPTP VPN (TCP/IP1723 fw and GRE on Zyxel). No VPN
Zyxel firewal, only NAT.

Parameters:
auth=MS CHAPv2, encr=MPPE 128, compression MPPC
>From LAN, the VPN is connected and autheticated
>From outside (from WAN) I'm not authenticated! with netstat the port
1723 on server is open (this mean the firewall settings are ok... I
suppose)
Any ideea about this?

Thanks a lot.