“Zimbra proves to be one of the most complete
email/collaboration packages in the world of open source.
Nevertheless, it does have its idiosyncrasies that can make it a
challenging product to use. On more than one occasion, I
encountered duplicated contacts and duplicated appointments when
attempting to synchronize with calendars and contacts hosted on
Google. Other times, the application would lock up, requiring a
shut down and in extreme cases, a reboot of the system. However,
those problems were relatively rare, but still worth noting.“Of course, several other open source and free
email/collaberation applications are readily available, but the
three mentioned above prove to be of the most interest. That is not
to say that others, such as Claws Mail, Sylpheed and Eudora OSE
have no value, but are perhaps best suited for niche users that
require something different than the traditional Outlook
experience. “