authd .. was a kick ass mail filter for its time. Unfortunately, spammers eventually got smarter, and better applications (such as SpamAssassin) have become increasingly neccesary. The authd.org site has details on the history behind it, as well as the last public release of it.
Authd’s primary goal was to refuse mail before even seeing the message body. The state of mail these days, is that .. you really need to look at the message bodies any more before determining ham or spam. as such, authd any more is nothing more than an alternative ACL system for SMTP, and hardly a spam deterrent.