When garbage becomes a problem: Blacklisting

Everyone is familiar with the phenomenon that led to 'blacklisting': that myriad of annoying spam emails that clog up your inbox in the morning. To ward off this unwanted garbage, DNS-based blackhole lists (DNSBL) were created, which match used addresses with suspected cases in real time (with the 'Real Time Blackhole Lists' - RBL). The Domain Name System (DNS) is the information system that can deliver messages to the exact computer required.

A DNSBL therefore keeps a worldwide list of those computers whose addresses in the Internet Protocol (IP) have already attracted unpleasant attention as spammers. As long as the global avalanche only affected computers whose sole purpose in life was to send spam emails, this system worked perfectly and there were hardly any false positives.

Nowadays, however, mass emails are being sent from a fixed location less and less often. In most cases, 'Trojanized computers' are involved in the big game of 'Viagra', 'alleged lottery winnings' or 'strange inheritances from Nigeria'. A malicious program ('Trojan') has then, unnoticed by the owner of the computer, crept onto the hard disk and uses its hardware as a spam slinger.

This is also the problem with blacklisting. In order to be removed from a blacklist ('delisting'), a great deal of effort is required, which often requires a lot of money and nerves. It therefore always depends on which DNSBLs are used by the provider's mail server.

Unfortunately, many providers do not delist for the reasons mentioned above.