Zimbra: Restrict sending to certain domains

http://wiki.zimbra.com/index.php?title=Restrict_sending_to_certain_domains

GUIDE: Postifx: HOW TO: Multiple access lists for protected Distribution-lists
http://www.zimbra.com/forums/administrators/15041-guide-postifx-how-multiple-access-lists-protected-distribution-lists.html 

Restrict users to certain domain

http://wiki.zimbra.com/index.php?title=Restrict_users_to_certain_domain

http://ubuntuforums.org/showthread.php?t=827581

Zimbra Tips : Restricted User to Sending Email for Certain Domain Only

http://vavai.net/2008/11/02/zimbra-tips-how-to-restricted-user-to-sending-email-for-specified-domain-only/