13.4 Exploder Mailing Lists

When mailing lists get extremely large, they sometimes include the names of other lists at other sites as recipients. Those other lists are called exploder lists because they cause the size of a list (the number of recipients) to explode. For example, consider the situation in Figure 13-1.

Figure 13-1. Exploding a mailing list

This figure shows that a message sent to nuts@oreilly.com will, in addition to its list of users, also be forwarded to allgov@wash.dc.gov and ads@uu.uu.net. But each recipient is also a mailing list. Like the original nuts list, they deliver to ordinary users and forward to other sites' mailing lists.

Unless exploding lists such as this are correctly managed, problems that are both mysterious and difficult to solve can arise. A bad address in one of the distant exploding lists, for example, can cause a delivery error at a remote exploder site. If this happens, it is possible that the error notification will be sent to either the original list maintainer or (worse) the original submitter, although neither is in a position to correct such errors.

To ensure that error notification is sent to the person who is best able to handle the error, mailing list entries in the aliases file should be set up like the following. It is an approach well-suited for exploder sites:

list:           :include:/path/to/rebroadcast.list
list-request:   list-request@original.site, local maintainer's address here
owner-list:     local maintainer's address here

Here, list is the name of the mailing list that explodes mail by sending the incoming message to the users listed in rebroadcast.list. Note that the envelope of the outgoing message will contain the address of a local user, one able to fix problems in rebroadcast.list. Messages to list-request will be relayed to both list-request@original.site and a local user, thus delivering administrative mail to the originating list maintainer and to the local maintainer, one of whom should be able to handle the request.