In an ideal world people would only send messages to a mailing list in order to communicate useful information to people on that mailing list. Unfortunately in the real world people have an unpleasant habit of posting unwanted messages to mailing lists. This normally happens when people reply to a message and copy their response to the list by accident. However there has been an unpleasant and increasing trend for people to send unsolicited junk email to mailing lists in order to promote some product or event. This can be extremely disruptive with large mailing lists, especially when the list members do not understand why they are receiving these messages. To help combat this threat, several different types of mailing list exist, each type targeted at a different audience. The types of list are Open, OpenCam, Closed and Moderated. An additional setting can be made to allow emergency moderation of all messages (Secure).
Anyone, anywhere in the world can send a message to an open list. This clearly has high potential for abuse. It should only be used for widely published contact addresses, for example a list set up to handle undergraduate admissions queries would clearly need to be an open list. List managers should restrict list membership to a small number of responsible individuals who understand that junk mail can be sent to an open mailing list.
Opencam (Open for Cambridge)
People with Cambridge email addresses (that is, an email address that ends in cam.ac.uk) can post to an Opencam list. So can the list managers, moderators (if the list manager has nominated any) and anyone on the authorised sender list. Messages from other (i.e. unauthorised) senders are sent to the list managers or moderators for further attention. This is a slightly more secure state than a straight open list. It is principally designed to stop unwanted junk email from being sent to the list by people outside Cambridge.
Only list members, managers, authorised senders and any nominated list moderators can post messages to a closed list. This is the natural state for a small discussion list where a few dozen people want to communicate freely with each other, but don't want unwelcome interruptions from people outside the list.
Messages from unauthorised senders are forwarded to the list managers or moderators. The moderators/managers can then take appropriate action (e.g. approve, discard) via the web interface (a link is included in the message they receive). Alternatively it is possible to approve posting simply by replying to the confirmation message. A message is also sent back to the originator of the message to explain the situation.
One problem with closed lists is that people often post messages using different email addresses. Mailman has been amended for @lists to know about the common registration scheme (that is, it knows that firstname.lastname@example.org and email@example.com are the same person). It also extracts information from the managed mail domains so that it knows firstname.lastname@example.org is the same as email@example.com. However, it cannot make the same judgements about other mail domains, even mail domains within Cambridge.
Also known as "restricted".
Only list managers/moderators and a nominated set of authorised senders are allowed to post messages. If no moderators have been set up, the list managers automatically become moderators. Any other messages sent to the list must be explicitly approved by one of the list moderators. This is the natural state for very large mailing lists used for announcements (e.g. society mailing lists). Messages from unauthorised senders are forwarded to the list managers/moderators who can then take action as outlined in the section on "closed" lists above. A message is also sent back to the originator of the message to explain the situation.
This can be set after the list has been set up, and is done from the list's main options page. List managers can either select Secure from the pull-down posting status menu or can check the yes button for the Emergency moderation of all list traffic option.
Every message sent to the list is forwarded to the list managers/moderators even if the message appears to originate from a list manager or moderator in the first place. A second message may be sent back to the originator of the message to explain the situation. Note that any list of authorised senders is ignored when the status is set to secure: all messages must be explicitly approved by a list moderator.
At face value there would seem to be no advantage in using a secure mailing list in preference to a normal moderated (restricted) list and substantially more work is involved for the list moderators. However, it is an unfortunate fact of life that it is very easy for people to forge email messages. If someone insists on trying to post messages to a closed or moderated (restricted) list by forging messages as a list moderator, the list manager can simply switch the list into secure mode until the problem has been dealt with. This is preferable to shutting the list down entirely. Secure lists are a point of last resort and rely on the fact that while it is easy to forge messages as if they came from a list moderator it is fortunately rather more difficult to intercept messages that are sent by mailman to the list moderators.