Email in Zimbra:
The email application allows users to activate the Zimlets "request read receipt". For me, it's a brand new and useful support for email services, which I have not yet experienced elsewhere.
Besides, the interface is very convenient for busy people while scanning the incoming messages/emails and at the same time opening them (3 specific columns/areas of the main "Mail" tab).
Despite sticking with Gmail for over a decade in terms of business, Zimbra have impressed me with some other aspects as well... If the time comes, I would prefer Zimbra to my current solution.
Zimbra mail reminds me of Hotmail, which I did employ in the past. And comparing it to other email free services like Yahoo, Gmail, etc., Zimbra is still on the way to getting in touch with a wider audience, such as: employers, employees, educators, officers, etc., because it has been a symbol of the open-source community and such a powerful resource for students and educators to explore further, as well as to contribute.
Calendar, tasks in Zimbra:
Different than other applications I have been using for these functions, namely: calendar, tasks, document, etc. Zimbra simply saves time, as these are attached to the main interface by a tab instead of a link to a different page. So, it's recommended for businessmen, especially.