Distribution List

Hi All,

I have a Zimbra-4.5 server(open source) running on CentOS 4.4 final. I have an issue related to distribution lists. I have created one distribution list and added users in the list. But there is no option for restricting users (ie, for receiving only and both sending and receiving). I need to give authorization to only few users for sending mails to the distribution list. All other users must be read only users. How can i get this working with zimbra -4.5.

Regards,

Premoddev