The ucspi-tcp package contains a set of tools to accept, reject, or conditionally accept mail using rules that key on the IP address or rDNS of the remote site. Some rules are locally defined and used only on a single host, while others are shared among multiple hosts. The standard way to handle shared rules is via DNS blacklists or blocklists (DNSBLs, either way). Local IP and rDNS rules are handled by tcpserver, using a rule file created by tcprules. This is the same rule file we set up in Chapter 8 to distinguish between local injection hosts and remote relay hosts. DNSBLs are handled by rblsmtpd, which runs between tcpserver and qmail-smtpd.