Hmm, I have always been against allowing non-subscribers to post toWithout moderation I'd agree. But my Yahoo Groups that operate this way in fact receive very few non-member attempted postings (sometimes months between them), either spam or valid. In Yahoo's group settings you get this effect with the combination of allowing non-member posting and selecting "Moderated" (for all messages) or "Moderated for new members".
My use case for this ability is relatively small groups - classmate groups, PTA groups, and family groups, typically with no more than a few hundred members, all of whom I know in person. I almost certainly would not elect this option for a large, public, general-interest group; probably not in any group with open membership.
What about an option so that messages from non-subscribers just getThe only advantage I can think of to routing it to +owner is that if it is spam I delete it and I'm done. Routed to a pending queue I have to forward the pending notice to the +reject address and then delete it.
Even so, I prefer that such messages go into the Pending queue, or perhaps into a Spam? queue, so that I can approve them if appropriate. My typical use case is a member who has changed email providers without updating their subscription, or accidentally used the wrong one today.
Now, some additional automation to help them correct the error would be welcome (but hardly an urgent need). Something like a "you used an address that is not subscribed to this list" return message that allows them to verify that address as a new subscription. I would only select that action in cases where I am sure from the message body that I'm dealing with someone who is or should be a member. And of course this only applies to groups that elect to allow non-member posting - in other groups a straight bounce of the attempted non-member post is appropriate.