A pipe transport is a mail-delivery mechanism that sends incoming message data into a local command or program instead of handing it to a mailbox, SMTP peer, or file. In mail servers such as Exim, this is often used for filtering, automation, or custom delivery handling.
It matters because a pipe transport can turn message content into command input, so any unsafe expansion, argument handling, or privilege setting can become a security problem. If a delivery rule runs with elevated rights, attackers may try to influence the command, inject data, or reach unintended code paths. Defenders review pipe-based rules carefully, restrict what data is expanded into commands, and avoid privileged execution unless it is strictly required. In practice, mail systems treat pipe transports, .forward files, and features like force_command as sensitive trust boundaries.