Many thanks to Chris, Duane and Bruce for their very helpful posts, and apologies for the slow response. And I am embarrassed to say that, after examining our Activity Log (at Chris's suggestion) I can now see that I was mistaken. Although the non-member did in fact "Reply All" to the message that was copied to our group, his message WAS NOT in fact received by our members. Confusingly, the non-member also blind copied in a (non groups.io) list, of which many of our members are also subscribed. So to an untrained eye (like mine) it looked like his message had breached our wall, when in fact his inclusion of our groups.io list as a recipient would not have got through to us.?
I have also checked our Message Policies, on Bruce's suggestion, and indeed the first checkbox was already unchecked.
So, false alarm. Please accept my apologies.
Barry
