[ Thanks to Jostein
Berntsen for this link. ]
“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.”