While I was thinking about Nick's suggestion for mod_rbl (blacklist lookups with mod_smtpd) I happened upon this idea, which is somewhat unrelated to the smtp project.
DNSBLs, the dominant form of real time blacklisting, are not specific to SMTP because this is just a way to publish lists of IP addresses. RHSBLs, which look up the address in an SMTP envelope, are specific to SMTP however. Apache -- the HTTP side too -- would benefit from DNSBL support. Or does this already do this? For example, both the CBL and AHBL projects list IP addresses of hosts engaging in activities such as proxy hijacking and spam relaying. This means it would be useful for webmasters to be able to make use of the published DNSBL to deny access to http requests. Because DNSBLs are an efficient way to publish lists, webmasters might start using a DNSBL lookup feature in Apache to limit abuse of say message forums, cgi scripts, proxy gateways. Currently, this has to be done by importing a complete list of IP address (often tens of megabytes) into a firewall script or Apache configuration.
