[Richard]

I guess I posted too quickly. The somewhat obvious work around is to create another Zimbra account which either does not receive email or which forwards it to the Administrator's account. The calendar for that account becomes the Administrator's calendar and is shared with the Secretary's and the Administrator's real accounts. The Secretary can safely sync to that account without seeing any of the Administrator's email when using Outlook.

I'd still be interested in any other solution, but I feel much better knowing I have a workable solution.