If your clients continue to use Outlook, be sure to upgrade them to Office 2007 SP2 (or at least the February Outlook 2007 rollup) to avoid a number of annoying bugs.
Zimbra wiki: Outlook 2007 February 2009 cumulative update and Office 2007 SP2
SP2 is behind schedule (this is not unusual, nor is it unique to Microsoft), but is currently expected to ship April 28th.