The PMDF configuration file is the heart of the PMDF configuration. This file establishes, via channel definitions and domain rewriting rules, what types of connections the PMDF system has to other systems and mailers, what addresses are local to the PMDF system or local to the PMDF site, and how to route corresponding messages, optionally altering addresses in the process. The types of connections to other systems and mailers are established by means of the unifying PMDF construct, the channel. The altering of addresses and the routing of corresponding messages to channels are controlled by the domain rewriting rules.

Operationally, when a message enters the PMDF system it must be placed into the proper channel queue or queues. PMDF's message enqueue routines consult the PMDF configuration file to determine the proper channel queues. Each recipient address is processed through the domain rewriting rules to determine to which of the defined channels to enqueue the message. In addition, domain rewriting rules may also modify addresses; for instance, the message's addresses must be rewritten to eliminate any reference to local system nicknames or aliases (called short-form names in RFC 822) since these are not allowed in outgoing messages. All information about local short-form names and how to eliminate them via rewriting is usually contained in the configuration file or a subsidiary database such as the domain database discussed elsewhere.

On OpenVMS systems, the configuration file is pointed at by the logical PMDF_CONFIG_FILE;¹ on UNIX systems the configuration file name and location are specified by the PMDF_CONFIG_FILE² setting in the PMDF tailor file; on NT systems the configuration file name and location are specified by the PMDF_CONFIG_FILE³ NT Registry entry.

The PMDF configuration file is an ASCII text file that can be created or changed with a text editor. The configuration file should be world readable. Failure to make the configuration file world readable may cause unexpected PMDF failures.