Zimbra DesktopMicrosoft OutlookMozilla Thunderbird
Cross PlatformMicrosoft Windows, Mac OS, LinuxMicrosoft Windows OnlyMicrosoft Windows, Mac OS, Linux and more
CostFreeCommercial LicenseFree
Open Source100%No100%
Support Enterprise Collaboration ServerZimbra Collaboration ServerMicrosoft Exchange ServerNo
Identical UI as Enterprise Collaboration Server Web UIYesNoNo
Integrated CalendarYesYesVia Lightning extension, beta version
Corporate Address BookYesYesNo
BriefcaseYesNoNo
TasksYesYesVia Lightning extension, beta version
Groupware Sharing and DelegationYesYesNo
Gmail Mail SupportYesYesYes
Google Address Book SupportYesNoNo
Google Calendar SupportYesNoNo
Yahoo! Mail SupportYesNoVia WebMail extension, not production quality
Yahoo! Address Book SupportYesNoNo
Yahoo! Calendar SupportYesNoNo
POP3 SupportYesYesYes
IMAP4 SupportYesYesYes
Cross-account Message ViewYesNoNo
Cross-account Calendar ViewYesNoNo
Cross-account Address BookYesNoNo
Email Address Auto CompleteYesYesOnly previously mailed contacts
Drag and Drop from DesktopYesNoNo
Support Tagging All Data TypesYesNoNo
Conversation View Across FoldersYesNoNo
Usable When OfflineYesYesYes
Seamless Online/Offline TransitionYesYesNo
Access All Data OfflineYesDoes not download all data by defaultDoes not download all data by default
Manage Multi-GB MailboxesYesSlowSlow
Search across Multi-GB DataIndex based, fastLimited and slowLimited and slow
New Mail NotificationYesLimitedLimited
Themes and ColorsYesNoYes
Auto UpdateYesYesYes
ExtensibilityZimletsPluginsExtensions