Our mail system allows incoming e-mail messages to be fed directly, upon receipt, to a custom user-specified program. Typical actions performed by such programs include:

• Forwarding the message (or a rewritten version of it) to one or more arbitrary recipients.
• Responding automatically to the recipient with some message (e.g., vacation(1)).
• Updating some database.
• Writing the message into an arbitrary file or maildir(5).

User-specified programs designed to process e-mail must be as quick and as efficient as possible (C/C++ applications or Perl scripts are best, PHP scripts are generally not recommended). They must also be carefully written to avoid potential security problems, or dangerous mail loops (although our system has built-in safeguards against mail loops).

Under traditional sendmail-based installations, mail-to-program commands would be specified in the ~/.forward file. On our servers, the commands must be specified on a per e-mail address basis (or, if multiple addresses should deliver to a same commands, macros can be used).