you need to check your recipe! you should never open port 137,138,139 to any machine from the internet.
PPTP uses TCP 1723 and GRE (protocol 47)
ports 137-139 are opening HUGE HOLES in your network security. Especially if it is to a Microsoft server, esentialy you have told people to open their windows shares to the World. NEVER OPEN these ports.
I don't coment on much but this is bad networking practices at their worst.