msgfilter

msgfilter [-q] [command [argument ...]]

-q
    Optional. Suppresses verbose output or status messages from the filter. Its availability depends on the specific implementation of msgfilter.

command [argument ...]
    The program to execute after the message has been filtered. The filtered message is typically passed to this command via its standard input. If no command is specified, the filtered message is usually written to msgfilter's standard output.

The msgfilter command is not a universally standardized Linux utility. Instead, it typically refers to a program or script designed to intercept and process email messages within a Mail Transfer Agent (MTA) system, such as Qmail or Postfix. A prominent example is qmail-msgfilter, an utility often used in Qmail installations.

Its primary function is to read a complete email message from standard input, apply various filtering rules (e.g., spam detection, virus scanning, content modification), and then either pass the (potentially modified) message to another program for further processing (like queueing for delivery), or discard it, or reject it with an error. The actual filtering logic is often implemented by external scripts or internal code within the msgfilter program itself, making it a flexible entry point for custom mail processing workflows.

The functionality and exact options of msgfilter vary significantly as it is often a custom script or an application-specific utility rather than a standard system command. Its use is typically confined to specific mail server setups (e.g., Qmail). Misconfiguration can lead to mail delivery issues or security vulnerabilities.

In MTAs like Qmail, msgfilter is typically invoked by the daemon responsible for receiving mail (e.g., qmail-smtpd) to process messages before they are handed off to the internal mail queueing system (qmail-queue). This allows for real-time filtering of incoming email.

The core strength of a msgfilter utility lies in its extensibility. Users or administrators can implement custom filtering logic using scripting languages (e.g., Perl, Python, Shell scripts) or compiled programs, allowing for diverse applications such as spam blocking, virus scanning, content archiving, or header modification.

The concept of message filtering programs like msgfilter gained prominence with the rise of MTAs requiring customizable message processing capabilities. qmail-msgfilter, for instance, has been a part of the Qmail distribution since its early versions, serving as a template for users to implement their own filtering logic.

qmail-queue(8), qmail-smtpd(8), procmail(1), sendmail(8), postfix(1)