mxallowd is a daemon for pf/pflog which implements a slightly improved
nolisting mechanism. Basically your nameserver has to be configured to
return two MX ip addresses of which one does not run a mail server on
port 25 (the one with higher priority). Most spammers try to connect
directly to the first mailserver -- mxallowd blocks that. You have to
connect to the first one and then to the second one, direct connections
do not work. Real mailservers (not spammers) have to try all MX ip
addresses in order (sorted by priority) until they succeed in delivering
the mail.
