Please do
zmcontrol -v as the Zimbra user on your server and enter the output into your profile.
You may find this helpful:
Free Busy Interop for Exchange - Zimbra :: Wiki
For sharing your calendar with users outside of the local ZCS, you can create a share that works over the web. Just right-click on the calendar, choose "share calendar", and go from there. Note that the address of an external user (as requested in the dialog box) is just used to deliver a notification and to act as the username for authentication purposes, so you could put anything in there and then choose the option not to send a notification.
Note that the shared calendar data can be viewed in either html or .ics format. Outlook should be able to work with one or the other. The instructions for internet calendar subscriptions
here seem to be what you're looking for.