What I have discovered on further research is the non-member is a member, but the email address they used to be a member, is not the email address they used to post to the group.
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The post was made to the group using the non-member email address, along with the responses, in the Messages page.
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In Activity it said "Non-member ... attempted to send message ... via email". For the members that responded it says ¡°sent message¡via email¡±
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The group does not allow non-subscriber posts.
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There is only one other moderator, not one of the people that responded, and Activity does not show that action. The group does not have email integration, and I receive all messages from the group.
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I did search the All Activity tab of the group's Activity log for that non-subscriber email address, and only the "Non-member ... attempted to send message ... via email" showed up.
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Since you mention looking in my Spam folder, I decided to do that. Sure enough all the posts were there (I¡¯ve never had a member¡¯s post go into my Spam folder before), and it looks like one of the members forwarded an email by a non-member. I need to ask that member if that is what they did.