It comes with most features you've come to expect, including email address autocompletion, drag-and-drop and keyboard interfaces, filtering support, and many others, and it can easily be extended with additional plugins.
It integrates with other online accounts like Facebook, Twitter, Google, and Dropbox for a more connected experience, and it also renders HTML emails very well compared to some other clients I've used, which can struggle with complex markup.
For more on Zimbra, see this article from Zimbra's Olivier Thierry, who shares a good deal more about Zimbra's role in the open source community.
Zimbra's web client is licensed under a Common Public Attribution License, and the server code is available under GPLv2.
I have to admit that I'm most familiar with an older version of Zimbra, which felt at times slow and clunky, especially on mobile, but it appears that more recent versions have overcome these issues and provide a snappy, clean interface regardless of the device you are using.
A desktop client is also available for those who prefer a more native experience.
The original program was derived from two others: an intra-machine email program (SENDMSG) and an experimental file transfer program (CPYNET) (:amk:irh:) Project Gutenberg is started by Michael Hart with the purpose of making copyright-free works, including books, electronically available.In fact, there are a number of open source alternatives available for those who want more freedom, and occasionally, a completely different approach to managing their email without relying on a desktop client.You'll still need an email server to use with these clients.It features a drag-and-drop interface that generally feels modern and fast, and comes with a slew of features: canned responses, spell checking, translation into over 70 languages, a templating system, tight address book integration, and many more.It also features a pluggable API for creating extensions.