Hey,
What OS and Office you running?
It may be linked to the rule in Outlook that runs as default which wipes all rules on startup, In outlook...
1. In the Navigation Pane (Navigation Pane: The column on the left side of the Outlook window that includes panes such as Shortcuts or Mail and the shortcuts or folders within each pane. Click a folder to show the items in the folder.), click Mail.
2. On the Tools menu, click Rules and Alerts.
3. If you have more than one e-mail account, in the Apply changes to this folder list, click the Inbox you want.
4. In the list of rules, see if any of them refer to deleting on startup im sure its in there somewhere, if it is.... If not the tech guys may want to see your zimbra log.
5. Click Delete.
If you need info on getting a zimbra log look here.
Outlook Troubleshooting Options - Zimbra :: Wiki
Hope that helps
Simon