What Does Spam and Open Relay Blocking System Mean?
The Spam and Open Relay Blocking System (SORBS) is an anti-spam project that inspects servers from which emails are received to check whether the emails were sent from proxy or open-relay servers. SORBS was developed in 2002 and shared with the public the following year.
Techopedia Explains Spam and Open Relay Blocking System
SORBS tests systems to ascertain whether they contain open proxies, open relays, etc. (Open relay servers are configured to permit anyone on the Internet to send emails via the server.) SORBS aims not to exploit servers during the testing process. It does not actually block websites or emails, but rather lists their hostnames.