Beginning April 1st, the Mailman listservs started failing to deliver messages to their membership. This happened because one of the real-time blackhole lists that Mailman was subscribed to went out of service permanently, causing Mailman’s mail server (sendmail) to reject all messages outgoing to the list memberships. So the messages were received by Mailman and archived properly, but they were not delivered. This was found and resolved on April 7th.
This coincided with another email problem on April 2-4, which masked the listserv issue, which is why it took so long to find and resolve.