Are you trying to be able to go to my.domain.com to access it through a browser? Or is doing a connect to server from within finder preferable/sufficient?
Assuming its the latter, you would need to open port 548 on your firewall. Create a user with access to that share and then the client from out in the WAN would from Finder select:
Go >> Connect to Server...
Put in your IP address
Authenticate
It always helps if you can provide as much detail as possible in what you are trying to accomplish so that we can provide the most accurate advice.