Question: What is DNSBL and should I care about it?
Answer: DNSBL stands for DNS Block List or Blacklist. Essentially, it is a published list of ipp addresses that are in some way connected to spam. Although the most common form of DNSBL is a list of actually servers and/or computers that actually send spam, there are some that flag ip addresses based on association with spamming computers or the potential to send spam. The latter two are particularly controversial.
The reason you should care about DNSBL is that there is always the possibility that your server could end up on one. If you run a dedicated server with multiple users, the potential is greater because you could actually have users who abuse their privileges and send spam. As a result, their ip address will be flagged, and if that ip address is shared with others, everyone suffers.
The result is that any other email servers that rely on the lists will reject email sent from your server. You can prevent this by ensuring that your server is locked down and not an open relay for spam. Furthermore, you can periodically check the most common block lists to see if your ip addresses are listed. If they are, find out how spam is being sent through your server and eliminate the problem. Within 48 hours of fixing the problem, most lists will remove your IP addresses.