What Peter didn't say is that this problem seems only to afflict () account holders with email addresses, ...
?
So until some better solution is in place, it seems that an immediate triage would be for owners of an unmoderated group to put all subscribers on moderation.
This way their errant messages to +owner and etc. will be caught in the Pending list rather than sent out to all the users.
That's not too hard to do. In the Members list use the Search box to find all btinternet subscriptions, checkmark them all (with the box next to Display Name), then select Change Moderation in the Actions menu. In the pop-up Set Moderation Status dialog select Override: moderated then click the Change button. Repeat if there is more than one page of search result.
That could, of course, includes attempts to join by email but as far as
I can see there aren't any in that category in the Activity Log for the
group I moderate.
No, there wouldn't be.
Those would have been logged as "Non-member ... attempted to send message ... via email" instead. And
ironically
the would-be member would have received back the bounce message that their message was rejected because they are not a member.
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