I found the solution! It's a simple one, but I guess noone knew exactly what I was talking about.
Basically, I needed to NIC cards: one to communicated directly within the network and a second one to connect directly to the internet. With this solution, I still maintain connectivity with the internal network (file sharing, etc.) and also avoid the problem of not being able to send internet mail from within the network.
Originally Posted by falko
Is it maybe a problem with your router not allowing loopbacks into your network?
Or maybe a firewall setting?