For whatever reason, I've had trouble configuring Mailman 2.1.x to either reject or discard attempted postings by non-members to mailing lists.

Sometimes it's a member using a different email address, but the majority of the time it's spam or wrongly directed email. So after a fair amount of poking around, I found a command-line way of doing the deed here.

To summarize, if your list is called "foo" you can do this to export the config to a file:

config_list -o foo foo
vi foo

Then find the string generic_nonmember_action = 1 and change the 1 to a 2 (reject) or 3 (discard).

Then re-import the configuration:

config_list -i foo foo

And you should be good to go.

Now if I forget this again in the future, at least I know I can come back to my own blog to search for it. :-)