Zimbra Collaboration is light to manage. It works on the Linux operating system, which is far better to manage than Microsoft Windows. Microsoft Exchange works only on the Windows server. We prefer working on the Linux server for security reasons and ease of management.
Zimbra Collaboration also has some integrated tools for security. It some modules to fight malware. These modules are not very powerful but are still doing a great job. Microsoft Exchange doesn't have a basic module. You have to purchase a security solution and add it to the solution.
The cost of Zimbra Collaboration is one more reason for liking it. It is also very simple to migrate from Zimbra Collaboration to another email solution.