Procmail
Procmail is a mail delivery agent (MDA) capable of sorting incoming mail into various directories and filtering out spam messages. Procmail is widely used on Unix-based systems and stable, but no longer maintained; users who wish a maintained program are advised to use an alternative MDA, such as maildrop.
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.
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.
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
- Martin McCarthy, The Procmail Companion, Addison-Wesley, S. 256, November 2001, ISBN 0-201-73790-6