I'm new to Zimbra and have a few questions. Basically I already have a postfix / dovecot / cyrus based mail system built which has been working for the last few years but I want to throw in a web based layer with calendar / tasks functionality. Appears Zimbra is the choice but I want to customize the front end to suite my enterprise environment.

So which of the following is more difficult?
1) Install Zimbra and just use it's core functions but completely re-design / write the web front end? Or write the web interface from the ground up?
2) Write the web front end from scratch building on my existing setup, adding in calendar / task / contact, etc.

I guess it depends whether Zimbra has everything exposed via SOAP API and I can just use Java to call?

Also, is the source that is available the full network edition or only contain the feature in the open source edition? This is important because I'm using Outlook at the moment and if the code is only the open source edition this means it won't work with Outlook.