miltersexternal

Miltersexternal refers to milters that operate as external processes or services separate from the main mail transfer agent (MTA). They implement the Milter protocol to inspect and potentially modify messages during SMTP transactions. External milters can reside on the same host or remotely, communicating over Unix domain sockets or TCP, and are typically designed to be started, monitored, and scaled independently of the MTA core.

Architecture-wise, the MTA connects to external milters for specific filtering steps. The milters participate in a defined negotiation of capabilities and message events, such as SMTP dialogue, header processing, and the DATA phase. They can return commands to continue, reject, or defer a message, and may add, remove, or modify headers, content, or recipients. Milters may be stateless or stateful and are usually run as dedicated daemon processes, offering isolation and separate lifecycle management from the MTA.

Configuration and operation involve registering external milter endpoints in the MTA’s settings. The MTA negotiates protocol version and capabilities with each milter, and transport security (such as TLS) or trusted-network authentication may be used for inter-process or network communication. Performance considerations include parallel milter connections, timeouts, and fault handling to prevent delays if a milter becomes unavailable.

Use cases for miltersexternal include modular spam and malware filtering, content policy enforcement, and header or metadata inspection. Benefits include easier maintenance, scalable deployment, and the ability to leverage specialized filtering services; drawbacks include added latency and the need for robust reliability and security controls.