Some estimates suggest that up to 80% of all mail flowing throughout the internet is spam. This has become a significant problem for users and a troubling problem for mail server administrators. One approach is to dynamically look at email content and try and determine from the content whether the email is legitimate mail or spam. This approach will identify spam emails but invariably will sometimes mistake legitimate email for spam. This side effect tends to annoy users of the mail system even more than the spam did in the first place.

Another approach is to use spam and open relay databases maintained on the internet to reject mail from black listed mail servers. These databases are maintained by organizations like www.spamhaus.org and www.ordb.org. They are actually DNS servers populated with "A" records of known spammers and mail servers functioning as open relays.

What is an open relay? A mail server that will accept email from anyone and deliver it to users at domains that the mail server isn't intended to serve is an open relay. Open relays are used extensively by spammers.

Most mail servers can be configured to check these lists when receiving mail from the internet. If the IP address of the smtpd client is in any of the databases specified then the client is rejected.

Here is an example of how to configure Postfix to consult a couple of these databases: 


From the postfix main.cf file:

maps_rbl_domains = relays.ordb.org, sbl-xbl.spamhaus.org
smtpd_client_restrictions =   permit_mynetworks, reject_maps_rbl, check_relay_domains


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