Directory Assistance + LDAP Domino-Group

Hi, I need help. I want to send smtp mails to an domino-server (dmz) with smtp authentication. For authentication I want to ask an internal Domino ldap-server. Only persons in a special domino-group should be authorized for authentication. For this I will use DA, but how can I accomplish this? Which entry are needed to ask for a member in group “smtpsender”. regards walter