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.