Blacklists for email servers

Blacklists for email servers

As you are no doubt aware, spam is an ever increasing problem on the net. A good deal of it is sent through sites called "open relays." These sites will accept email from users not on their site for users also not on theit site and forward it to them. Back in the days when the Internet was new, some sites could not send email directly to other sites or were not continuously connected to the net, so this was a "friendly" thing to do. However, now there is really no need for this friendly behavoir and it is used mainly by spammers to obscure where the mail is originating. Or worse, they can deliver a single email to an open relay and address it to hundreds of recipients, thereby causing the open relay to do most of their work. So while it was once good to be an open relay, now it has become a tool for spammers to multiply their effective delivery capacity.

To defend against a good deal of incoming spam, and to put pressure on these open relays to reconfigure their servers so they cannot be used as a tool for spammers,, the server through which most email for the CS faculty servers passes, refuses email from open relays. Such email will be rejected with a message something like

   Your message did not reach some or all of the intended recipients.
       Subject:    RE: New Classes
       Sent:    2/18/02 5:40 PM
   The following recipient(s) could not be reached:
       'Recipient Name' on 2/18/02 5:40 PM
             There was a SMTP communication problem with the recipient's email
   server.  Please contact your system administrator.
             < #5.5.0 smtp;591  your host is
   blacklisted by Send your questions to>
As you see, the name of the failed recipient is identified as well as the hostname of the service identifying the sending system as an open relay. (Connecting to that host on the web and looking up the originating host address in their database will get you to a place to get further information.) And if all else fails, a local address is supplied for an end user to contact for further assistance.

"But someone cannot contact me now!" Suppose you hear from someone trying to get in touch with you. Perhaps they do not understand the message and/or do not know how to proceed. You can refer them to a more verbose, user-friendly explanation of what has happened and how to remedy things at They will also be directed to that explanation if they email the "questions" email address in the bounce message.

This page last updated 1/25/2007.