PMDF is a general-purpose, store-and-forward system for distributing computer-based mail. The term store-and-forward means that PMDF automatically handles the requeuing and retransmission of mail messages necessitated when network links or services are temporarily unavailable. In contrast to mail user agents (MUAs) such as VMS MAIL or Pine which are used to create and read electronic mail messages, PMDF is a mail transport agent (MTA) responsible for directing messages to the appropriate network transport and ensuring reliable delivery over that transport.

PMDF provides a uniform distribution environment that can be interfaced to multiple user interfaces (MUAs), networks, protocols, and transport mechanisms. As this interfacing, from the user's point of view, is accomplished transparently, PMDF presents to the user a homogeneous mail network; i.e., PMDF seamlessly blends heterogeneous mail networks into a single, coherent mail system.

This chapter presents a very brief sketch of the internal structure and operation of PMDF, including an overview of the PMDF configuration file. The subsequent two chapters describe in detail this configuration file, the heart of PMDF: the domain rewriting rules used to rewrite addresses and route mail, and the channel/host table which establishes a site's available channels (message paths and gateways) and their characteristics. Subsequent chapters provide additional details on other facets of PMDF.