1. Login to Webmail interface through URL http://mail.yourdomainame.com 

2. Click on the Calendar icon on top of the bar menu.

3. Click on button and choose Add To Outlook.

4. Fill up Outlook Display Name column.

5. Launch Application tab will pop up and Choose Outlook.

6. Click Advanced...

7. You may rename Folder Name (Calendar name) and fill up Description.Once done, click OK

8. Once done, you will see your calendar on outlook. It will sync with a server.