If your site is behind a firewall, you might find that you need to configure an HTTP proxy to be able to fetch the new syndicated sites over the internet.
This proxy must allow outgoing requests without any authentication.
In your site configuration (in the section labelled "SPIP features"), enter the proxy in the following format:
http://proxyname:port/
where proxyname is the name of the server acting as a proxy, and port is the number of the TCP port (most frequently 3128, 8080, or even 80) which will handle the requests.
Warning: This set-up is global: SPIP will fetch all the syndicated sites through this proxy. Since SPIP 1.9, it has been possible to specify that certain hosts should not use this proxy. But for any prior versions of SPIP or if you need more precise configurations, you will need to contact your own network administrator directly.