Use VI to edit /usr/syno/mailstation/etc/main.cf
Find the line
mydestination = $myhostname (this will be the domain you entered in the gui)
For additional domains, just add them to this line, ie:
mydestination = $myhostname mydomain1.com mydomain2.com
Note: Every user will receive mail for all domain names, so johndoe@mydomain1.com, johndoe@mydomain2.com, etc.
Thanks for the info.. work fine indeed.