Hmm... Let's see...

AXIGEN - Office Edition
Licensing - Free (registration required! - why?)
Mailboxes - max. 9
Domains - only 1
Support - Free for 1 year
Webmail Client - Rather schematic
Basic search - Yes
Advanced search - Unknown
Tags - Unknown
Sharing - Unknown
Calendar - Unknown
Conversations - Unknown
Resource/Group scheduling - Unknown
Documents - Unknown

ZIMBRA - Open Source Edition
Licensing - Free (under open source licensing terms)
Mailboxes - unlimited
Domains - Unlimited
Support - Community based
Webmail Client - Outlook like
Basic search - Yes
Advanced search - Rich "Search Builder" interface for creating and saving advanced, multi-condition searches
Tags - Ability to quickly categorize items by attaching "Tags" with user-defined names and colors
Sharing - Ability to share contacts, calendars, and documents with internal or external users
Calendar - AJAX Calendar - Rich, interactive Calendar functionality using only a web browser
Conversations - Organized views of message threads to streamline user Inbox management
Resource/Group Scheduling - Ability to schedule meetings, book resources, and evaluate and manage free/busy information
Documents - AJAX Documents - Ability to create rich web-based documents and wikis

My point is...
Maybe this is not the best place to promote a commercial product.