Zimbra is an open source email server. It does email, calendar, contacts, and various other useful things that software like Microsoft Exchange does.
And there is a guide at bluish coder:
The problem is that Zimbra has a number of dependencies on other open source projects and they don't provide direct downloads for those dependencies.
This means you spend a lot of time tracking down what software is needed (OpenLDAP, MySQL, etc) and need to get the specific versions required by Zimbra.