On the Corrs mailing list, there have been sometimes mentions of censorship, and that it should not happen, and also discussion of moderation and whether it should be used. Here are a few thoughts I have on the matter.
In my view, the concept of censorship does not really apply to the mailing list. Censorship is used to prevent voicing of opinions and ideas which are believed to be "bad". Moderation, on the other hand, means preventing messages (a form of opinions and ideas too) which the majority of the audience does not want to hear. There is a difference here.
The mailing list is quite different from real life. I do hold a firm view that censorship in general is bad, and believe in free speech. (This issue is actually not as clear cut as one might imagine, because censorship can also be "good" depending on what your views are, but getting into that is not the point here.) However, the concept of free speech is mostly irrelevant in the discussions on the mailing list.
Everyone is free to start a mailing list about any particular topic on the Internet. (Everyone might not be capable of it, but that's a personal limitation, not a limitation imposed by the environment, so it's not censorship.) This is what free speech, and freedom from censorship is about -- you may discuss anything you like.
However, when a mailing list is created with the purpose of discussing some
particular topic, this means that the people who take part have a special
interest in that topic, and so messages on other topics are not welcome.
While freedom of speech means people should be allowed to discuss any topic,
it does not mean that they can do so anywhere. Posting off-topic
messages on topic-dedicated mailing lists is very inconsiderate of other
users of the list. Claiming that it should be allowed due to freedom of
speech is a fallacy, because freedom of speech does not entitle anyone to
be rude towards others. The old truth is that
freedom of speech does not mean that you can yell 'Fire!' in
a full movie theater.
The members of the mailing list should be free to discuss their chosen topic without getting disturbed by off-topic postings. Preventing off-topic messages is moderation, not censorship.
The difference between censorship and moderation is that in censorship opinions are prevented from getting expressed because they are believed to be harmful to everyone else. In moderation, the reason for stopping some opinions from being voiced is because the audience does not wish to hear them.
All of the above I've written is just discussing things on the principal level. There are many practical problems in applying moderation in a fair way -- the biggest being who determines if a message is on-topic of off-topic for a mailing list? Opinions on that will vary. In practice, mailing list moderation (the traditional, pre-screening type of moderation at least) is carried out by individuals who try to determine what the audience would consider on-topic or off-topic. This method inherently cannot be absolutely right since it's the the individuals who perform moderation based on their personal opinions and judgements, not the audience itself. Still, experience on the Internet has shown that this can lead to very good and satisfactory results.
There are also other kinds of moderation, for example self moderation (used on an "unmoderated list", where it is asked that members do not post off-topic messages) and post moderation (asking people who post off-topic messages to not to do so in the future). These are nowhere near as successfull in preventing off-topic posts as pre-screening, but they are also a lot less drastic measures.