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ezmlm/idx lists by default(*) reject messages that do not have the list address in the ‘To:’ or ‘Cc:’ headers. Most “bulk E-mail” does not pass this test. If you still receive too much junk mail to your list, you can set up your list as a subscriber-only(*) list. This creates a small overhead of users who try to post from addresses that are not their subscriber address. See above on how to deal with this.
Since the message sender is easy to fake, this is not secure. A determined spammer can subscribe to the list or just send mail ‘From:’ a subscriber address. The next level of countermeasure is to have a message moderated(*) (and possibly subscription moderated(*)) list. The overhead is higher - someone has to approve each message - but it is much more secure. ezmlm-idx has support for multiple moderators making it easy to set up a list with fast and efficient moderation, which is a very reliable way to keep junk mail off a list.