Anti-Spam-Daemon using nolisting/iptables

 mxallowd is a daemon for linux/netfilter(iptables) which uses nolisting
 (http://nolisting.org). That means, you define two MX-servers in your
 nameserver and mxallowd will block the access to any of these mailservers if
 the client did not try the other one. This blocks non-RFC-compliant mailers,
 which are often in use by spammers and so-called direct-to-second-mx which is
 also a technique mainly used by spammers.