Zimbra is an open source email messaging and collaboration application.

The powerful Ajax web client integrates email, contacts, shared calendar and online document authoring into a rich browser-based interface.

Full support is provided for standards-based APIs (IMAP / POP / iCal / CalDAV) as well as MAPI and iSync, which enable seamless compatibility with other clients like Microsoft Outlook, Apple desktop suite, and Mozilla Thunderbird.

Intuitive Message Management and Search

Conversation View hides redundant messages that clutter up your inbox
Search Builder is a simple "WYSIWYG" tool to make advanced queries; save favorite searches as "virtual" message folders
"Tagging" automatically highlights email from important people
You can view attachments (like Word or Acrobat files) immediately as HTML instead of downloading and opening them in another applications
Address Book

Supports multiple Address Books (personal and server-side Global Address List)
Easily share Address Books and contacts with peers
Autocomplete from personal and/or Global Address List (GAL)
Create personal distribution lists
Robust Personal and Shared Calendaring

View multiple schedules as 'overlays' on your calendar, one click to toggle them on and off
Complete resource and group scheduling, with delegated access
Share and publish calendars with peers
Subscribe to remote calendars as iCal feeds
Online Documents

Create, share, and publish rich Documents online
WYSIWYG editor makes it easy to add spreadsheets, images, etc.
Turn Documents into collaborative Wiki pages
First application to use the innovative AJAX Linking and Embedding (ALE) technology Zimbra pioneered
Easy Sharing and Publishing

Simple sharing model common to Address Book, Calendar, and Documents
Share internally (read and/or write access) or publish externally with a simple URL (read only).
POP Account Aggregation

Set up multiple external email accounts to be viewed and managed within ZCS
Email replies from different accounts can have different Signatures and Mail Identities
Voice Integration

When used in conjunction with a VOIP gateway such as Asterisk, users can quickly launch calls, conference calls and to access voice mail from within the Zimbra Collaboration Suite
Productivity Enhancing "Mash-ups"

Use Zimlets to get single click access to calendar, contacts and maps.
Easily create new Zimlets for your unique needs
View Mail in Basic HTML Mode

Basic mode with a non-Ajax web UI for poorer connections, legacy browsers or easier accessibility
Personalisation

Users can choose their own colour themes
Administrators may quickly re-brand and/or create new themes
Benefits of Commercial Version

Zimbra Collaboration Suite (ZCS) Network Edition features enhancements not available in ZCS Open Source Edition:

Support from Zimbra. Access to Support Portal for email, and phone crisis support plus file and track feature enhancements
Desktop client compatibility. Sync mail, contacts, and calendar to Microsoft Outlook and/or to Apple (Mail, Address Book, iCal)
Professional administration. Real time mailbox backup and restore, high availability clustering, storage cost management
Zimbra Mobile. Over-the-air synchronization of mail, contacts, and calendar data with mobile devices.
Advanced web productivity. Ability to search for content inside attachments and view attachments as HTML instead of downloading
Domain management. Ability to re-brand the web client and administer multiple customer domains
Zimbra Technology

Fast, easy install - all dependent components are packaged together in one simple installer
Proven open source - built with Linux, Apache Tomcat, Postfix, MySQL, OpenLDAP, Lucene
Uses industry standard open protocols - SMTP, LMTP, SOAP, XML, IMAP, POP, iCal, CalDAV
Efficiency and horizontal scalability - Zimbra server is 3-5x faster than comparable systems, plus each server has its own data store, message store, and set mailbox accounts. To scale simply add more servers