SPAMHAUS blocklists: http://www.spamhaus.org/
NJABL dnsbl blocklist: http://www.njabl.org/
DSBL blocklist: http://dsbl.org/
VIRBL blocklist: http://virbl.bit.nl/
Why this webpage?You are probably here because an e-mail you sent to a CWI user was bounced by our mail system with an error message such as "error: 550 Mail from ... refused based upon information obtained through ...". This is because your mail host is a source of spam, or is open for passing on spam from third parties, has been informed about this, and has not taken (timely) action to prevent it.
What is spam and how is it distributed?Spam, also known as UBE (Unsolicited Bulk E-mail) or UCE (Unsolicited Commercial E-mail), is increasingly becoming a nuisance and a threat to the Internet. Spam steals both system and human resources. CWI has contacts throughout the world and so our e-mail addresses are known world-wide; as a consequence massive amounts of spam are daily send to CWI. This has forced us to take necessary measures to both block spam and prevent our own mail hosts from being used to pass spam on to others.
Spam is distributed in several ways, but the most-used method is to use a relay host: a spam-generating computer contacts an external mail host and asks that host to forward (relay) their mail to the given destination. Even on today's Internet relay hosts are commonplace, but technical measures can be taken to prevent them from relaying mail for third parties. Spammers search out mail hosts that are not protected against third-party relaying for their nefarious deeds. What this means is that these open relay hosts are carriers of a lot of spam: if e-mail arrives from them, there is a good chance that it is spam.
Blocking spamIn order to identify these spam-carrying hosts, four blocklists are currently used by CWI. If someone discovers (usually by receiving spam) that a host is an open relay, they can report that host. The system administrators of that host are then contacted and asked to switch off third-party mail relaying. If within a reasonable time the administrator does not disable the relaying, their host is put on a blocklist of potential spam-generating sites, and other mail hosts are then at liberty to use that list to refuse e-mail from such sites.
This is not a single-handed CWI action: many mail hosts have adopted blocklists as a defense against spam, and because of the success of the system many more are adopting them. CWI currently uses the blocklists that are given under "References" at the top of this page.
How to get off a blocklist?The downside of using blocklists is that bona fide mailers from blocked sites cannot reach us. The solution is to tell your system administrator or service provider that your site is a source of spam or open to third-party (spam) relaying, and should disable relaying immediately. Once done, it's only a matter of hours to get off a blocklist.
For real emergencies, CWI has a whitelist through which the above mentioned blockings are bypassed.