A perl script that filters email based on a list of known good addresses. You list the addresses you wish to accept email from, and all others are save to a staging folder which can be purged at will. New in version 1.3, gatekeeper can automatically accept email from any addresses you’ve sent email to, and new in version 1.4, it can use rbl style services to bypass even staging a black-listed message. gatekeeper is 110% effective at blocking unwanted emails.

Note that this is for use with unix systems that use unix mbox style mailboxes, not maildirs or anything else.

Thanks to Don Teske for the help with the concept.