Normally only access to the destination ports 21 and 80, as well as to 443 for SSL are allowed via the proxy. The reason for this is that the proxy's CONNECT function can be generally used for protocols other than HTTP and therefore any firewall restrictions could be bypassed. Some web servers use other ports (such as 81, 8080, etc.). If the clients should be able to use them, they must be entered in theor under Services > Proxy > Settings.
Normally, the proxy accesses the requested servers directly over the Internet. However, it is also possible that the Intra2net system forwards all requests to another proxy (Parent-Proxy). This can be configured under Network > Provider > Profiles : Services. This means that a different proxy can be used for each provider.