Ok, your server handles this as local email address. This can have several reasons:
a) The hostname of the server is ***.com
b) You use ***.com somewhere in postfix main.cf, e.g. as mydomain or myhostname value
c) ***.com is in /etc/mailname
d) A Email domain ***.com has been created in ISPConfig
e) ***.com is used in /etc/hosts, e.g. for the IP of the server.