The mailing list, a form of group communications pioneered in the nonprofit days of the Internet, is being reborn in commercial form. Across the network, sites have sprung up that host lists free of charge for companies and consumers, with ads bringing in the revenue.

The concept of mailing lists is simple: Each member of a list can send e-mail messages that all others automatically receive. Any member can answer the message with one that also goes automatically to everyone else. Setting up the hardware and software to accomplish this can be tedious; hence, the free hosting services.

Lists number in the hundreds of thousands today and cover subjects as diverse as the world itself -- marketing, compost gardening, geopolitics, development of toiletries, family gossip. Some are "moderated," meaning that a list member looks over incoming messages and decides which are worth sending to members. Unmoderated lists proceed without oversight. Some lists are private, by invitation only, linking members of a company's sales department, perhaps. Others are open and can be joined by anyone who finds out about them.

Site operators hope to make money from the fact that most lists have specific subjects, and therefore advertisers can use them to target people likely to buy their products. A fly-fishing list, for instance, would presumably be a good place to advertise rods. Ads are typically tacked on to the end of messages that subscribers receive.

Here are three major names in this field. All let you create and search for lists. Beyond that, features often vary.

EGroups.com (www.egroups.com)

It claims more than 5 million members taking part in 200,000 groups. About 95 percent of its messages are delivered within 50 seconds, the service says.

Basic service is free, but messages will go out without ads in them if list owners pay $4.95 a month. A special package for businesses for $49.95 a month provides special services such as tech support and help in setting up elaborate corporate lists. It also removes the ads.

A photo album feature lets people post pictures to be viewed by other members of their groups. It also has a group chat service, allowing people who sign on at the same time to send "real-time" messages back and forth. Lists get 20 megabytes of storage space.

Onelist.com (www.onelist.com)

It reports it has more than 4 million members and 160,000 lists that generate 19 million messages a day. Each list get 5 megabytes of storage space, which list members can access individually and download -- sales information, for instance, or recipes from a cooking list. Calendar feature helps people schedule events and send out reminders.

A survey feature allows list managers to poll list members -- thumbs up or down on opening a new office in Reston, say, or on "The Phantom Menace."

List managers can cut out ads from the messages by paying $4.95 a month. They also can have customized sig tacked onto the end of each message.

Topica (www.topica.com)

In April, it acquired the Lizst directory, a four-year-old compilation of lists all over the world. It also has exclusive relationships with other third-party list directories. Through the Topica site, users can locate and subscribe to a total of about 50,000 lists, including ones that aren't hosted by Topica. It does not disclose numbers of users.

Topica has no ad-free option, but at present puts only one-line ads for its own service on the message traffic. It plans a service in which list owners will be able to insert ads into messages or operate lists that consist entirely of specialized advertising.

HOW THE E-MAIL LIST WORKS

When a user sends out a file to everyone on a particular e-mail list, it does not go directly to each address. The file is sent first to a central server, which then sends a copy of the file to each address on the list.

Server

Recipient

Recipient

Recipient

When a recipient responds to the e-mail, it goes back to the server, which then sends a copy of that file to each address on the list.

Server

Sender

Recipient

Recipient

Recipient