For one of lists, one of the subscribers had the strangest problem. He kept getting bounces that the list alias does not even exist, whilst at the same time other subscribers could well write to that list. It turns out that his mail server was rewriting the To:-address, resolving the CNAME of the hostname that is responsible for running the lists. As we run a virtual-domain based mail server, this was not a smart choice: Whilst the email address does indeed exist on the CNAME‑d host (lists.xiqit.de), it does not exist on the fully qualified name the server listens to. So we added an MX record for the hostname that also points to the ‚primary‘ name of the server (the one which the RDNS also resolves to), and now all is well. HIs email does reach the list.