DNSBLs are DNS Black Lists. They are lists of IP addresses known for spamming or which are part of known spammy networks. As most spam these days is generated by malware infested Windows PCs, some blacklists are simply lists of consumer grade broadband IP addresses, such as your home ADSL or cable connection and are registered by the ISPs which own them. Consequently, running a mail server on a consumer grade connection is not ideal as you may well get blocked. My IP address at home is listed by SORBS due to being a consumer grade line registered by my ISP with SORBS.
Commonly used DNSBLs inlude:
There are many more. You can get more information on them from their respective home pages. I stopped bl.spamcop.org when I found it was rejecting Yahoo's mail servers, which made it kind of pointless to me.
The idea is that you configure your mail server to call out to one or more blacklists when it receives a mail. It checks the list and refuses the mail if the sender's IP address is listed in the blacklist.
Below is some config to add it your Exim configuration if you are not already using DNSBLs, place it in your Exim ACLs.
deny message = Your email server is spam blacklisted at $dnslist_domain\n$dnslist_text !authenticated = * !senders = @@lsearch;/usr/exim/sender_whitelist dnslists = zen.spamhaus.org:list.dsbl.org
Here I am allowing users who can authenticate (I use SMTP authentication) and those email addresses which are locally whitelisted in the sender_whitelist file.