This has the advantage that it doesn't require any work for the list owner. The disadvantage is that the list owner loses control over who can participate the list's discussions and leaves the list open to potential spammers . A closed list does not accept any subscription requests. Attempts to subscribe to closed lists are returned to the sender with a message indicating that the list is not accepting subscribers. Lists that are created for a very specific purpose and that have a finite set of participants are often configured as closed lists.
Most lists allow anyone to post to the list .
That is, it is not always necessary to be subscribed to a list in order to send a message. Such lists are called open lists. A closed list is one in which only those who are subscribed can submit messages to the list. A closed list reduces spamming because potential spammers must first go to the trouble of joining a list. The moderator approves or discards the messages.