This could be a firewall problem. Can you use FTP from any other machine in the company to an outside ip address? If not then it’s probably down to a firewall or network configuration blocking your FTP traffic.
***added by Labnuke99***
The other thing to check is routing. Be sure that there is a default gateway defined for the subnet on the AS/400 and that you can ping or tracert to the outside host. Your pings or tracert might not work all the way to the destination but it should give you an idea if the traffic is even leaving your network.