Does it matter where the request comes from? Anyone can send post- requests to the mailman page (subscribers, bots, hackers etc) and mailman should not fail in any case IMHO.
Can you explain exactly what the scenario is that triggers this error?
This is the minimal example I could find: curl https://mail.example.com/cgi-bin/mailman/subscribe/news -d "language='" -H "Content-Type: text/plain;charset=UTF-8" You can of course add as many other data as you want.