This article contains some notes on mailing lists.
A fairly common practice is to put a fixed tag in the subject line of any postings that go through a mailing list. The idea is to make it easy for readers to group the messages received via the list separately from other emails they receive. But in fact, this kind method of grouping is a bad idea. Why?
Imagine that a reader replies to a poster to the list directly, i.e. as a private email. Since the message they are replying to has been through the list the subject line will have had a tag added. If the receiver groups messages on the subject line then this message will be grouped with the mailing list rather than with mail sent to them directly.
Instead, it is better to filter mailing lists on some header added by the mailing list software. In some cases a Resent-From field will be added; some add an X-Mailing-List field, which is even more convenient.
An even better alternative is to subscribe to each mailing list via a unique address, and use that information to handle the messages from each differently. This is only practical for users who can have enough different addresses routed to themselves, though, which is not everybody. Many modern mail transport agents do support some kind of suffix syntax; for example, I can receive richard+<anything>@sfere.greenend.org.uk.
[Note that this kind of subject-line tagging isn't the same thing as the subject-line tagging found in some newsgroups (and perhaps some mailing lists); that's something done by individuals to broadly categorize articles, rather than something enforced by the software. It's out of scope for this article.]
This is the practice of adding a Reply-To header to any message sent via the list to direct replies back to it. It's not necessary (because mail clients have a "group reply" or "followup" command), subverts the usual meaning in mail and news of "reply" and breaks any other attempted use of Reply-To. (Sometimes users don't notice that this breakage has happened, and send to a list a message they intended to be private.)
See ``Reply-To'' Munging Considered Harmful and Reply-To Munging Considered Useful for further argument on this subject.
One point that neither addresses is that Reply-To munging breaks cross-posting between lists (since it causes to a message received by one list to be sent to only that list rather than the complete set of original lists). Not all mailing lists allow cross-posting, of course, but Reply-To munging doesn't actually prevent it - it just makes it inconvenient, potentially creating half-cross-posted threads that are impossible for some readers to follow properly.
Lists operators who want to prohibit cross-posting with their list should either use a working technical measure to prevent it, or (more likely) admit that they can't prevent it by technical means and use social mechanisms instead.
(Apparently some people think that no mailing lists should allow cross-posting. I don't know where they get this idea from; it's obviously a question of individual list policy.)
The problem that Reply-To munging is usually intended to solve is that a sequence of followups on a mailing list tends to gather the addresses of participants in the thread. This means that they all get two copies of the message, one via the list and one directly. Some people dislike this, especially if they've lost interest in that thread.
Some people also like it, though, for various reasons; for example it may mean that they get the followups to their messages faster than they would otherwise. Clearly, then, this preference needs to be expressed by individual users, rather than imposed across an entire mailing list.
Mail-Followup-To and Mail-Reply-To suggests a new header to work around the problem. For much the same reasons as mailing lists shouldn't munge Reply-To, they shouldn't munge Mail-Followup-To either.
(There's an internet draft describing Mail-Followup-To as well, but it has expired.)
Some mailing lists limit postings to subscribers. Usually (but not always) this is intended to prevent spam being sent via the list, which is a reasonable enough goal.
However this can cause some problems. A common practice is to subscribe a mail-to-news gateway to the list (instead of subscribing directly to it), and thus read the list in a newsgroup instead. Often the newsgroup has limited distribution. This is very convenient: newsreaders often have a much better interface for reading mailing lists than mail clients do.
But, especially if the newsgroup is accessed by many people, the effect is that some readers (and therefore potential posters) are not actually included in the list of email addresses that it is distributed to.
Also, some people may read a mailing list via its web archive. This is really the same case as with a newsgroup, only in a different medium.
A related problem is that some people subscribe a unique address to the list for filtering purposes (see above), but post to it using their "normal" address. Similarly, some people want to use spam-traps on mailing lists.
Obviously, all of these things interact badly with a simple restriction of posting rights to subscribers. You could characterize the underlying mistake as a confusion between readership and distribution.
Some mailing list software attempts to improve the situation by allowing users to register a separate posting address alongside their distribution address. This is fine for those in the latter situation (subscribed but post with a different address) but no good for readers who aren't subscribed at all.
A solution which covers all of the above cases is to allow subscriptions that allow posting but do not send you any of the messages sent to the list. Both Majordomo and Mailman (at least) support this.