However I found that any of domain user logged in , the email address shown in roundcube (settings->identities)is like abc@localhost instead of the user’s domain name.
Virtualmin GPL contains some Install Scripts, including RoundCube.
My suggestion would be to try installing RoundCube using the Install Scripts (you’ll see a link for that on the left near the top when you log in as root), and to install RoundCube into one of your domains, and then see if things work properly for you with that.