Spam is a junk e-mail sent to numerous recipients. The general rule here is "The more the better". There is no wonder spammers use software for processing e-mails and so called "spammer lists". Suffering recipients have found a way to struggle, though. They decided to retaliate and now antispam war is the war of lists against lists. Spammer lists against Black lists.
DNSBL (Domain Name System Black Lists) based upon a regular DNS queries are probably one of the most popular and the most discussed weapon of antispam. The black list itself is a list of IP addresses of those who already proved themselves to be spammers. The mechanism of DNSBL is rather simple: unsolicited e-mails are blocked at the initial stage of SMTP connection and the recipient does not get an e-mail at all. In addition to spam filtering, systems using DNSBL lower the load of mail traffic.
There is still a possibility that important e-mails will not reach the destination, which is the main argument against black lists. It is easy to get on such list and be declared a spammer. Services that provide DNSBL support (e.g. block.blars.org, cbl.abuseat.org, t1.dnsbl.net.au) often have vague criteria for enlisting. Moreover, as soon as an IP address is listed as that of a spammer, one needs to present serious proofs of the opposite to be deleted from the list.
Choosing a certain service does not guarantee 100% protection. To get better results it is suggested to create own black lists (based on already available - www.declude.com è www.moensted.dk.) that will precisely correspond to one's needs. However, it is not an ideal solution as it has two major disadvantages. First, servers already offer hundreds of black lists (often for free). Is it really worth wasting time to make up a new one? Second, own lists hinder the work of the server seriously. In spite of the rapid development of DNSBL services spammers actually feel themselves free: they are much faster in finding new computers and IP addresses than those who make up black lists. Moreover, a part of servers providing DNSBL services, being free of charge in the beginning, now require payment (a nominal one, but still a payment) which lessens the number of those using such services.
Presently spam fighters found a more successful and flexible decision for mailing systems, so called "gray lists". Being a little milder than black, graylisting implies a treatment of letters from unknown senders with suspicion: these e-mails are not permanently deleted but received somewhat slower which gives an administrator opportunity to reverse a spam sentence.
Antispam campaign is so much like a ping-pong game. Black lists used to be a panacea, spammers got a respite - not for long, though. Now the floor is given to graylisting. Hopefully this would be the final hit. Who knows?