The mail server channel provides a group of automated server facilities that respond to commands sent as electronic mail messages. The basic idea is very simple. A user sends a mail message to the server requesting some type of service. The server responds by sending one or more response messages. The server can also update various local databases as part of the processing of some commands (e.g., mailing lists).

The mail server channel program presently provides three general types of services: distribution of general information about itself, file serving, and mailing list manipulation.

Distribution of general information means that the mail server will send general information about the server, in the form of mail messages, in response to requests for information. These requests are currently limited to the commands HELP, INDEX, INFO (a synonym for HELP), and LISTS.

File serving means that the mail server will process commands requesting files and will mail the requested files back to the requestor. The SEND command is used to request files and the DIRECTORY command can be used to find out what files are actually available. The ENCODING and MODE commands are used to specify how the file or files are read and encoded prior to being sent.

Mailing list manipulation means that the mail server will accept requests for subscription to or removal from various mailing lists that have been placed under the control of the mail server. The SUBSCRIBE and UNSUBSCRIBE commands perform the basic mailing list functions. The DIRECTORY/LIST command can be used to find out what lists are available, and the SEND/LIST command can be used to obtain a list of the current subscribers to a given list.⁵

It is important to keep in mind that MAILSERV merely handles files; in particular, MAILSERV can manage mailing list membership files. But MAILSERV has nothing to do with mailing list postings; postings to the list are handled by PMDF proper and not MAILSERV. MAILSERV merely manages subscriptions to and from the list and queries for a copy of the list. For a complete discussion on PMDF mailing lists, see Section 4.1.