Side-effects of improving CERN's spam filter
To further improve the fight against spam (unsolicited email), CERN implemented in June a mechanism that returns electronic mail messages to the sender if the originating server is wrongly configured. This "reverse DNS lookup" is an increasingly widespread technique to identify whether the sending e-mail server is legitimate and has a valid host name.
This change has nearly doubled the spam rejection efficiency at CERN. However, it has a known side-effect. A small number of correspondents are sending legitimate electronic mail to CERN from mail servers that are incorrectly configured (in technical terms, they do not have a reverse DNS registration). These persons will have their messages returned to them with an error message, provided their sender address is valid. The typical error message that the correspondent receives in such cases is "Relaying denied. No reverse DNS configured for IP".
If a correspondent complains to you that emails they are sending you at CERN are being returned with an error message, it is possible that the reverse DNS lookup is the cause. If so, you should suggest that they contact their email administrator, and point them to the following explanation on CERN's website:
http://cern.ch/mmms/Help/?kbid=171011
While this side-effect may prove inconvenient in the short term, the benefits to these correspondents of improving their mail servers will be considerable, not just for correspondence with CERN, but also for emails to the increasing number of organizations which are implementing similar measures.
The CERN Mail Services.