It most likely has nothing to do with FileMaker Pro, but rather, the firewall settings on your Airport router, and possibly your ISP. Some ISPs do not allow traffic to access ports below 1000 on their customers equipment, to prevent people from running their own FTP servers, mail servers, web servers, etc.
Thanks, I'll look into the firewall settings, however it's not the ISP (I've checked). If any other FM Pro/Mac users out there can help, re: firewall settings... please do so!
Thank you for your post.
You may have a conflict with port 80. Try specifying port 591, as that port is reserved by FileMaker. Then, have the external user try:
If the external user still cannot access the file, then you have a router/firewall issue that needs to be ironed out with your ISP.