Other files can be included in the primary alias file. A line of the following form directs the MTA to read the file-spec file:
The file specification must be a complete file path specification and the file must have the same protections as the primary alias file; for example, it must be world readable.
The contents of the included file are inserted into the alias file at its point of reference. The same effect can be achieved by replacing the reference to the included file with the file’s actual contents. The format of include files is identical to that of the primary alias file itself. Indeed, include files may themselves include other files. Up to three levels of include file nesting are allowed.