The multiple instance mode of the SMTP relay does not really work like a web server or a reverse-proxy.
Indeed, a Web server can share several domains with a single network address.
In the case of the multiple relay, you will have to create one network address per instance.
It is then necessary to manage several network cards or several virtual addresses.
For this, Artica allows you to create Virtual Interfaces or you can also use IP aliasing
SMTP Router” > “