I wrote:
Send mail to poster when their posting is held for approval? (Edit respond_to_post_requests) ... debatable....
Again, glad to discuss my reasoning. (It's late, and I've had too little sleep, or I'd say more here. But I've pretty firmly come down on the 'no' side after much pondering.)
As with many issues in mailing list administation, this issue partly involves the spam issue, and partly involves user psychology.
I'm sure you're acutely aware of what a blight on the Internet backscatter spam is. https://en.wikipedia.org/wiki/Backscatter_(email) This is an issue in a _huge_ variety of traditional SMTP infrastructure that is guilty of sending back autoresponses to claimed SMTP senders that are more often than not forged, making the infrastructure complicit in the spam problem.
E.g., any unpatched instance of DJB's qmail 1.03 (latest) is a horrifically bad netizen because Dan's qmail-smtpd process _first_ accepts incoming mail and only then decides whether to further process it for local users / MDAs / LDAs or whether to generate a non-delivery report (NDR) and outbound-deliver it to the (usually forged) claimed sender.
And -- my point -- Mailman sending 'Post by non-member to a members-only list' held-message notices will in a large percentage of cases be textbook backscatter spam. This will be true unless and until your accepting MTA is close to perfect in spam-rejection in its receiving stage.
I hate spam, which means I also hate being guilty of backscatter spam. Which means 'Send mail to poster when their posting is held for approval' is a terrible idea, and the correct setting is 'no'.
Separately from that, as I said, is the user psychology bit. But first I'll talk about the big picture, on my views concerning ethical mailing list administrative practices. My view is that ethics requires the listadmin to be responsive to _legitimate_ subscribers and aspiring subscribers (but not, say, to spammers), and to be transparent within certain limits and expectations. So, for example, a posting from a legitmate member should never just be discarded, because that is a failure of responsiveness to the subscriber. Just 'disappearing' an attempted post, e.g., by letting it expire out of queue or hitting the Discard button is, IMO, scummy behaviour. If it will not be approved, it should be explicitly rejected with a nice, concise notice about why.
Given that one is doing that, and doing it in a timely manner, is there a functional advantage to subscribers or the listadmin to also autosending an instant 'Your message has been held for moderator attention' notice? Not in my view, though I understand that some subscribers want maximal information instantly. Against that, there are the disadvantages:
Sometimes, with luck very rarely, you end up having good reason to suspect a current subscriber of inclination to commit misbehaviour in front of the mailing list as a large audience. So, you might want to quietly set that member's Moderated flag for a day or two, just so you can review what that particular hothead is saying before it goes out. If you're on the ball, the resulting admin-queue delay is so tiny that the subscriber probably won't even notice. Probably, after the first or second post without misbehaviour, you'll hit the checkbox on the Approve dialogue to clear the subscriber's Moderated flag again -- because your confidence has been restored and because a smart listadmin is a lazy listadmin. (You really don't _want_ to vet anyone's posts.)
Doing all of the above with the 'Send mail to poster when their posting is held for approval' feature off means it's all transparent to the subscriber and doesn't insult anyone or get anyone's back up.
There are probably other examples of this usefulness to _not_ having notices go out every time something lands in the admin queue, but I'm not remembering them at the moment. The larger point is; It's useful, to a degree that IMO massively outweighs some subscribers' desire to know instantly everything that's going on (especially if the listadmin acts on needed tasks in a timely fashion).
As an aside, I also strongly favour _not_ manually approving messages arriving from non-subscribed addresses. Instead, reject with a nice explanation that, in 2017, you have to post to mailing lists from your subscription address, but that people with multiple addresses they might wish to use for posting can still do so by subscribing _all_ their addresses and setting all but one's subscriptions to 'nomail'.
And also, you should almost never use the 'List of non-member addresses whose postings should be automatically accepted' feature (Privacy Options, Sender Filters), or its equivalent 'Add [$ADDRESS] to one of these sender filters: ' checkbox on the mail queue page. Why? Because any address in that roster gets prospectively exempted from _all_ posting rules, e.g., maximum message size for that mailing list, and unconditionally permitted to post. (The roster is thus a lurking menace to listadmins new to Mailman.)