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Spam and Open Relay Blocking System

(also SORBS)

Spam and open relay blocking system definition

The spam and open relay blocking system (SORBS) is a service that identifies and stores IP addresses known for sending spam. It also registers IP addresses that operate as open relays, allowing anyone to send emails through them without authentication. SORBS compiles these IP addresses into lists that email services use to filter out unwanted emails and reduce spam.

See also: spamming, spamware, open relay

How SORBS works

SORBS gathers data from multiple sources, like automated detection systems and user reports. It sorts the data into several types of lists:

  • Spam sources: IP addresses identified as spam senders.
  • Open relays: Servers that allow unauthenticated email relay.
  • Open proxies: Servers that act as intermediaries, used to mask the origin of spam.
  • Compromised systems: devices hijacked by malware to send spam.

When an email server receives a message, it checks the IP address of the sender against SORBS' lists. If the IP address is on any of the lists, the server can block the email or flag it as spam so it doesn’t reach the users’ inbox.


SORBS was created by Matthew Sullivan in 2002 to address the growing problem of email spam. GFI Software acquired SORBS in 2009 and continued to develop and maintain the system.