Allow all members to post to list without any moderation

Go to "Privacy options" –> [Sender filters] –> "By default, should new list member postings be moderated?" to "No" (default action).

There is a moderation (mod) flag in "membership management" to control postings from list members individually. When the flag is enabled, the list member postings will be moderated, and must be approved. It can be controlled for all members by "Set everyone's moderation bit, including those members not currently visible" in "Membership List" page.