Note: This info applies to Outlook profiles using the ZCS Connector for Outlook.
Using Outlook to access Free/Busy information from the Zimbra server
Navigate to the Free/Busy Options dialog:
Outlook -> Tools menu -> Options -> Calendar Options button -> Free/Busy Options button
If the hostname of your Zimbra server is domain.com, set the "Search Location" URL to the following (leaving %name%@%server% as the literal words "name" and "server" so that it will work for any account on the zimbra server):
Accessing Free/Busy information over SSL
Note that this will silently fail if you are using https without a valid ssl certificate on the server. It should be possible to cause Outlook to trust the self-signed CA cert from the Zimbra server by importing it into Outlook with this guide from Microsoft though there may be complications due to varying certificate formats.