Procmail

In email systems, procmail, a mail delivery agent (MDA), can sort incoming mail into various directories and filter out spam messages. Procmail is stable, but no longer maintained, and a number of security vulnerabilities have been discovered since its last release.  Users who wish to use a maintained program are advised by procmail's author, Philip Guenther, to use an alternative tool, because it has not been updated since 2001.

Replacements
Procmail was an early example of a mail filtering tool and language. Procmail is no longer maintained, yet it had just a single publicized security vulnerability since its maintainer Philip Gunther stopped maintaining it. Despite this it still remains in wide use as the security issues are not widely known, migration can be difficult, especially for those who have complex filters developed over years, and some see its capabilities as better than available alternatives.

 fdm (fetch, filter and deliver mail) is an ISC licensed replacement for fetchmail and procmail, written and maintained by an OpenBSD/tmux developer, originally written/released in 2006/2007
 dovecot-pigeonhole from Dovecot, an implementation of the Sieve language
 maildrop, included as part of Courier Mail Server, but available for use with other servers, too
 Sieve mail filtering language

Invocation
The procmail mail delivery agent is generally not started from the command line, but is usually invoked by mail delivery subsystems, such as a mail transport agent (like Sendmail or Postfix), or from a mail retrieval agent (such as fetchmail).
This makes the mail processing event-driven.  The companion tool formail allows procmail to be used in batch-processing on mail that already is in a user's mailbox.

Paradigm
Procmail is a data-driven programming language, similar to earlier line-oriented languages such as sed and AWK: it specifies conditions to match and actions to take on matching, in the form of "recipes". This approach has been followed by later languages such as maildrop and sieve.

Recipes
The procmail agent uses recipes, to determine where to deliver the various mail messages.

Elements of a recipe
Each recipe that procmail uses consists of:
 mode
 conditions
 action

Recipes can be conditional or unconditional
Recipes used by procmail can be conditional or unconditional. If the conditions are left out, the recipe is unconditional.

Types of recipes
Procmail has two kinds of recipes:
 Delivering recipes
 Non-delivering recipes

Processing of recipes
Recipes are read from top to bottom. The first delivering recipe terminates the delivery process (unless the mode flag specifies otherwise).

Conditions
Conditions are usually extended regular expressions, although there are other forms of condition also.

Basic operation
The procmail tool reads mail messages given to it from standard input. The procmail tool will process the recipes before distributing the mail messages into the appropriate mailboxes.

Pitfalls
By default, procmail will "bounce" undeliverable mail back to the sender. New users should be warned not to test new delivery recipes by running procmail on their existing mailboxes, since this could result in error messages being sent to all one's email contacts.

Other operations
Other common operations carried out with procmail include filtering and sorting of emails into different folders according to keywords in from, to, subject, text of the mail, or sending autoreplies, but more sophisticated operations
are also possible.

Spam filtering
A common practice is to let procmail call an external spam filter program, such as SpamAssassin. This method can allow for spam to be filtered or even deleted.

Managing mailing lists 
The procmail developers have built a mailing list manager called SmartList on top of procmail.

Further reading

See also 

 fdm (fetch, filter and deliver mail) — an ISC licensed replacement from an OpenBSD/tmux developer
 dovecot-pigeonhole — an implementation of the Sieve language for Dovecot
 maildrop — included as part of Courier Mail Server, but available for use with other servers, too
 Sieve mail filtering language

External links
Official website
Searchable procmail mailing list archives

References

Category:Mail delivery agents
Category:Cross-platform software