从网络外部上载时，生产服务器上的文件上载问题

We are having issue with a file upload functionality that we just deployed to our production server. That code works fine with our Dev and QA servers but when we deployed it to Production server we are having issues when uploading files from outside the network. It works fine when we do it from inside the network but as soon as we try to upload files from outside the network it doesn't upload anything and don’t give any error as well. Our application is developed in ASP.net 4 and we are using a Silverlight multi-file upload control to do the uploading process. On all the servers we have Windows server 2008 R2 and IIS 7. We have given the full rights to Network Services account on the folder and we are using port 80. Any suggestions on what else to check on the server or with the fire wall because I suspect it’s not the code issue because it’s working fine on all the servers except in one scenario when we upload files from outside the network on Production server.We are having issue with a file upload function