List

A one-way mailing list either accepts or sends information, but the user interacts only with the list server and not other users. Most one-way mailing lists are used for outgoing messages, such as announcements, newsletters, and advertising. The popular, but reviled, use of e-mail "spam" is an example of a one-way outgoing mailing list.

One-way incoming mailing lists are less common but still useful. Often, these are used to allow users to send commands to a server, which then takes appropriate action. For example, a fax-back service might accept incoming e-mail requests for a document, and it may fax (or e-mail) the requested document back to the individual. Even if the server returns data via e-mail, this type of mailing list is still considered one way because the user is not interacting with other mailing list subscribers.

A two-way mailing list lets users interact with other subscribers to that mailing list. This, the classic discussion mailing list, may be the most common type of mailing list (although perhaps not as common as spam), and it is certainly the predominate type of two-way mailing list. Two-way mailing lists are generally limited to subscribers, in the sense that a user has sent a message to a specified address to sign up for the list. List subscriptions may be open to the public, meaning that anyone can subscribe, or they may be private and restricted to known parties. The latter are often used for educational discussion lists for specific courses, as they enable an instructor to exchange messages limited to a known class of students.

Regardless of whether a two-way mailing list is public or restricted, it may be moderated or unmoderated. In a moderated mailing list, messages are reviewed by a central authority figure before being passed on to list subscribers. Moderation is often used to increase the "signal to noise" ratio of a list -- to select meaningful content and filter out uninformative comments, such as advertising or abuse. Of course, moderation requires human intervention, and lists with a high level of activity require more human time. Unmoderated lists pass messages from one subscriber to all subscribers without intervention.

List servers often support two modes of message delivery to subscribers. With real-time delivery, subscribers receive messages from other subscribers as soon as they are sent to the list server. This can result in a number of individual messages per day for each subscriber, which can range from a handful to hundreds or more depending on the activity level of the list.

A common alternative, digest delivery, summarizes all messages sent to the list over a period of time. When a subscriber requests digest delivery, the list server may send only one message per day (or per week) to the subscriber, that contains all messages from the list during that period. The latter method not only minimizes the number of individual messages received each day from the list server, but also limits the subscriber's opportunity to see or reply to time-sensitive messages.

Some list servers offer delivery modes beyond real-time and digest. Web-based delivery lets subscribers, or even nonsubscribers, view messages from list subscribers with a browser interface. Other advantages of Web-based delivery include navigable list archives and search facilities. Usenet delivery formats list messages as posts to a Usenet news server, allowing Usenet newsreaders (such as Fore Agent and Netscape Communicator) to access a mailing list. Especially for high-traffic lists, Usenet delivery eases navigation among many topics and messages.