includedir

includedir is a configuration directive used by Postfix to include the contents of all files within a specified directory into the main configuration file (main.cf). It enables modular configuration by allowing administrators to place related settings in separate files rather than editing a single large file.

When includedir is in effect, Postfix reads the files in the directory in lexical (sorted) order and treats their contents as if they were written directly in main.cf. The files should contain valid Postfix directives and can provide site-specific overrides or additional parameters. This approach is commonly used to manage configuration in a scalable way, such as separating default settings from local customizations.

Usage examples include lines like: includedir /etc/postfix/conf.d

Administrators may place multiple .cf files in that directory, with names arranged to control the order in which settings are applied. It is distinct from the include directive, which brings in a single file rather than all files in a directory.

Limitations and considerations: syntax errors in any included file can prevent Postfix from starting or reloading successfully. The includedir directive affects the runtime configuration, so changes typically require a reload or restart of Postfix. As with any modular configuration, clear naming and documentation help maintain readability and prevent conflicting settings. See also: include (Postfix directive).