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Manager Guide: "Post by non-member to a members-only list"
This may occur to legitimate list members if they use an email alias, or use one Internet Service Provider (ISP) to connect to the Internet, and another to send mail.
Solution: Check to make sure their "message-from" address matches the address they have subscribed to the list (check the internet headers of the message). If the addresses are different, try subscribing only the "message-from" address; or subscribe both addresses to the list, and disable message delivery to one of them.