When you have a mail server with the name mx123.thisserver.com, then with active DNS management, every domain you add to this machine will automatically have mx123.thisserver.com as default MX record that will be visible in the zone file or with this query
dig thisserver.com @localhost mx
Please change this imposing behaviour to default behaviour. A way to do it is by changing from "Custom DNS records" TAB to "DNS Records" and show there the proposed MX record editable. This would allow us to put a different mailserver per domain, more my idea of management.
Small exceptions like this now generate a lot of extra administration time or simply not using LC to manage the DNS at all. This cannot be the intention of the author.