I'm currently partly moderating mailing lists for Arch Linux. Since we've switched to mailman3 I receive *a lot* of info from the system, that mails are being held in the moderation queue due to being sent by non-members of the respective lists.
IMHO, we should configure all of our mailing lists to be subscription only (i.e. automatically reject all mail by non-members). It makes no sense for anyone to moderate these emails and is a huge waste of time for people dealing with the moderation of the lists (I get enough spam everyday as it is... ;-)).
While at it: To my knowledge, we are currently still allowing HTML emails to be sent to the lists and I think those should be automatically rejected as well, as it is unnecessary bloat and is not fitting our context at all (we are not trying to click-track users on a commercial info list).