Why this webpage?
Most likely you're here because an e-mail you sent to a CWI user was bounced by our mail system with a "Blocked relay", "Can't resolve relay's address" or "Can't resolve from-domain" error message. This is because your site or the relay host used is a source of spam, or because the relay host can't be trusted because its ip address has no domain name associated with it, or because the domain name used in the sender's address is not a registered domain name.

What is spam and how is it distributed?
Spam (also known as UCE, or 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 CWI has received massive amounts of spam. This has forced us to take necessary measures to both block spam and try as much as possible to prevent it from reaching us, in addition to preventing 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 an open 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 select 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. Furthermore, spammers often use e-mail addresses or ip addresses that don't exist.

It should be noted that self-respecting organisations and service providers too can fail to properly register ip addresses so they can be translated to domain names. It is however impossible to distinguish the "good guys" from the "bad guys". Such cases can be solved quickly though: ask your systems administrator to get it fixed right away.

Contacting CWI
To contact the CWI mail administrator from a blocked site, mail can be sent to postmaster@cwi.nl but only if the real user sending the mail is "postmaster" (an e-mail address that every site is required to have anyway).
For real emergencies, CWI has a whitelist through which the above mentioned blockings are bypassed. Entries on this list are only temporary: an entry is removed automatically after 3 weeks and will not be reinstated; if any spam reaches us through a whitelisted host within those 3 weeks, the entry is removed immediately and will not be reinstated.
To be put on the CWI whitelist, the system administrator should send e-mail (as above), specifying the work-/research relation with CWI, and contact person(s).

In conclusion
We hope you appreciate the need for anti-spam measures. In exchange for (we hope) a brief inconvience such as you now have, we regain a lot of lost resources and suffer a lot less inconvenience from spam.