I think you just need to create a correctly formatted .ics file as an attachment and send it. A very basic file would look like this...
Google for iCalendar Formatting and you'll find all sorts of other options you can throw in there for timezones, recurrence, whatever.
DESCRIPTION:Some Description Goes Here
Or even better...you can create the sample in your Zimbra calendar exactly the way you want it to appear to the end user, export your calendar to ICS, and look at the raw iCalendar data. Wrap the event in VEVENT and VCALENDAR ics tags and send it as an attachment...should work fine.