n this article, we're presenting "passthrough," a powerful technique for e-mail filtering I've never seen described by others. I particularly like it because it's so easy to "retrofit" on existing mail servers. I hope passthrough benefits you as much as it has my clients and me.
Here's the problem passthrough most often solves: A system administrator wants to filter a fraction of incoming e-mail, while handling the bulk of it as normal. The administrator knows a criterion by which he can discard spam, autorespond to information queries, log periodic receipts of updates, or otherwise process the selected item. How, though, can he or she extract only those items from the incoming message stream, and let all the others pass through for normal delivery?
Unix workers have a traditional solution. Most of the world's e-mail is handled at some point by industrial-strength Unix servers, where several distinct processes cooperate to route and deliver individual items. A typical combination has sendmail routing messages between servers, and procmail delivering messages to individual mailboxes on a particular server. Dave Anderson, president and chief executive officer of Sendmail, Inc., estimates, "Sendmail is the industry's dominant Internet messaging server, routing two-thirds of the world's email." Even Microsoft-based e-mail networks are often gatewayed through a sendmail-based firewall.
As a complement to sendmail, procmail is an 11-year-old open-source application that offers a wealth of filtering capabilities. It's already in wide use on large e-mail servers, and it's possible that the largest share of e-mail filtering on the Internet as a whole is currently performed by procmail.