Typically, the moderator screens out irrelevant messages, redundant or repetitive messages, misdirected messages, and personal flames. At the very least, the mail headers are stipped out, while at best, the digest manager also serves as moderator, filtering the contents. This is the most common form. Here, anything submitted to the list is distributed automatically with no filtering or human intervention. In some cases, only those who are subscribed to the list are permitted to submit items to that list.
Some sites maintain a master list of public mailing lists and newsgroups.
There exist many unpublished lists known only to their members. You join these by launching your favorite news reader and then subscribing to the list. In this case, send a mail message to the list manager. Most other lists are managed by a human list manager. The instructions retrieved by the list database typically contains the address of the list manager.