And what about:

select list_name, last_post_at from mailinglist;

Thank you, Odhiambo. I downloaded the smtp_report script. For some reason it doesn't report accurate numbers for my installation. The string it is searching for "smtp to <mailinglist>" appears a number of times for each post. In the end, I did what Ondřej suggested (in the next post):

select list_name, last_post_at from mailinglist;

And then to see how many posts each list received I did this:

select b.name,count(a.subject) from hyperkitty_email a, hyperkitty_mailinglist b where a.mailinglist_id = b.id group by b.name order by b.name;