inetd and smtpd

If you use an smtp server which runs from inetd (sendmail can operate this way, as can QMail and a number of other MTAs), here's a good way to do site-wide filtering using rblcheck; add the following to /etc/hosts.allow:

smtpd: ALL: spawn /usr/local/bin/rblcheck -q %a && \
	exec /usr/local/bin/smtpd || /bin/echo \
	"469 Connection refused. See http://maps.vix.com/rbl/\r\b\r\n"

This gives you RBL support on a site-wide basis, even if native support doesn't yet exist for your MTA of choice. This assumes that your inetd has support for tcp_wrappers checks; many Linux variants fall into this category, but other platforms may differ.

A word of warning, however: there are a number of very good reasons to not run a production mail server from inetd, which I won't elaborate on here. You might want to investigate more secure and reliable alternatives, such as tcpserver from D. J. Bernstein's excellent daemontools package.