Outlook 2003 SP2 adds some appointment regeneration code that isn't Zimbra friendly. You can create the following registry key to disable outlooks appt regeneration behavior.
Key: HKEY_CURRENT_USER\Software\Microsoft\Office\11.0\O utlook\Options\Calendar
DWORD: DisableMeetingRegeneration
Values: 1
There’s more info on the new functionality here:
http://support.microsoft.com/?id=899919
If you don't have SP2, you shouldn't see this problem. This is bug 7623.