Some time ago, I asked on the qmail discussion list if there was the mailing list archive in the standard mbox format, but the answer was negative. Good people on the list at least explained me what steps I needed to do to create the archive. As the mailing list is hosted on the ezmlm mailing list manager, I needed to pull each and every mail message from it, one by one. At a volume of more than 100000 messages it was very demanding, but eventually I did it.
To save other souls of doing such a daunting task again, I'm putting it all on this page for an easy download (archive splitted by year). Be warned, though, ezmlm did not return RFC822 compliant messages, so all the messages were first processed with a crude perl script which extracted original messages and added missing From_ headers, regenerating timestamps in it from the Date: header. That's the best idea I could come up with. The script is attached below (ezmlm-to-mbox).