Here's a little more discussion about how to link up two related lists (like a discussion list and announcements list). I still haven't really solved the problem, but this info may still be helpful to other folks trying to set up similar list-relationships... (Also, if there is some option or feature I missed that would do what I want, please tell me :-)

My objective is: "test-announce" members get only stuff sent to "test-announce" "test" members get stuff from both "test" and "test-announce" Restrict posting to "test-announce" to subscribers of either "test" or "test-announce" (this part will be hard) Restrict posting to "test" to "test" subscribers (but allow test-announce messages in too)

Unfortunately there really is no way to do what I want. Restricting posting privilege to members only really only works for that specific list and doesn't seem to be compatible with cascading lists...

A feature that might be cool for a future version of Mailman might be to allow adding to "posters" setting (ie. Addresses of members accepted for posting to this list without implicit approval requirement.) in terms of "Members of (x) other list also on this machine". For example, the test-announce list can allow unmoderated posting from its own members (member_posting_only = yes) and additionally from "test" members... where "posters" list might include something like "members(test)". That is, things that look like members(*) in the list are taken to mean "allow from subscribers of x" where x is some other list.

This could be useful for any list whose posting list is different from its actual first-tier subscriber list... for example, if you have a "test-administrators" list, you can quickly set it up so that test-administrators are allowed to send announcements to test-announce, but nobody else (and you won't have to maintain two lists).

Anyway, given the current feature list, here are the two ways I found to set up linked lists.

Option one: simple alias hack

People subscribe to either "test-announce" or "test". Messages sent to test-announce are copied (via tweaking their aliases) to both lists. Mailman interprets this as if a message was sent to "test-announce" and BCC'd to "test". ("test" must be told that "test-announce" is an alternate alias to avoid mail being held for "Implicit destination")

This option is not great, because in order for any member (of either list) to send to test-announce, BOTH lists need to be set open to all senders (member_posting_only must be off). Here's why:

From test member to test-announce: "test" copy goes through "test-announce" copy held for approval (unless "test-announce" is open to all senders) From test-announce member to test-announce:
"test-announce" copy goes through "test" copy held for approval (unless "test" is also open to all senders)

This option is easier to set up, and is great for smaller lists where outsiders are not as likely to try to spam the list (or you don't care if the odd external message makes it through). You would need to either open posting to everyone, hold all announcements for approval, or maintain a separate list of authorized senders to the announce list.

Option two: third "umbrella" list (as suggested by Ken M, with slight changes)

People subscribe to either "test-announce" or "test". Actual incoming mail for "test-announce" is really redirected to "test-umbrella". test-umbrella has two members, test and test-announce.

This only works if member_posting_only is turned off, and "test-umbrella-admin" is added as an authorized poster to both lists. Since the umbrella lists doesn't have any real people as members, turning on member_posting_only doesn't help... you still need to either open posting to everyone, hold all announcements for approval, or maintain a separate list of authorized senders to the announce list.

The advantage of option two is that you can leave the "discussion" list as restricted to its members. This would be better for a high-volume list that is unmoderated but doesn't take messages from non-members, except the occasional announcement (which you approve as they come or limit to a select few announcement-authorized people). In this model, members of the announce-only list don't have authority to send either announcements or discussion, unless you open the umbrella list to the entire world.