For anyone interested in private distribution lists and access controls tune into the discussion at the bugzilla page for bug 9620
Bug 9620 - Mail policies and access control for sending to distribution lists
This feature has long been neglected despite a number of repeat forum entries, numerous votes in favor of the feature, and repeated creation of duplicate tickets.
Workarounds, and config hacks that cannot withstand a simple update are NOT solutions - this is a standard feature in most any enterprise e-mail solution yet this issue has existed in the form of a feature request for over a year with little to no action, and no documented statement of intent by Zimbra one way or the other aside from a long, technical, and highly detailed howto on integrating mailman, a proverbial sledgehammer killing the fly.
Let Zimbra know you want enterprise level e-mail from management tools to basic features and let's not allow these items to stagnate!