Kenos MAILCOW_HOSTNAME=mail.domain.xyz
That is correct, so no need to change it.
But something is wrong with your installation. Please go through the docs again, and check everything regarding prerequisistes and installation. In 99.99% you missed on something.
I am assuming you are running on a supported Linux version and docker/docker compose version?
And check all the logs, especially those of postfix. Does it bind to port 25?