A Unix based Exchange replacement

If there's one area where Microsoft produces good software, then it must be their groupware and office product line. On my work, we're using the GUI nightmare called Lotus Notes, and I would trade everything for an Outlook based emailclient. On the sole condition that I would not have to administer it, of course : Outlook seems horrible to maintain, and it's virus prone. I'm still puzzled why there's no Unix alternative for Outlook. There's PHPGroupware, but many people will revolt against a pure webbased product (although it may seem strange in these Hotmail based times).

Rick Moen recently posted a list of Outlook replacements. Click on the Read More link below to see the full list. There tends to be confusion in these discussions because of lack of agreement on what the term "Exchange replacement" means. At one extreme, something qualfies only if it accepts Microsoft-proprietary RPC connections from MS-Outlook for MAPI transactions providing 100% of
the functions the Outlook / Exchange Server combination du jour supports. At the other extreme, Web-based access (e.g., Sherpath) and
glorified BBSes (First Class, Citadel/UX) are deemed worthy of consideration.
Anyhow, here's a list I maintain as part of http://linuxmafia.com/~rick/linux-info/groupware [linuxmafia.com]:

• MS Exchange Server (server end; NT only), MS Outlook (client
  end; Win32, MacOS). Very limited support of open-protocol clients
  (IMAP, webmail?). Microsoft Corp. wants to sell you Exchange 2000,
  these days, but Exchange 5.5 is still very common.
• Lotus Notes / Domino (server end, Linux supported), Lotus Notes (client end; Win32, MacOS). Limited webmail access (iNotes).
• Novell Groupwise. http://www.novell.com/products/groupwise/

  [novell.com] Server end runs on either Novell NetWare 5/6 or WinNT.
  Client end is proprietary Win32 client or webmail. A native Linux
  client is under development.

• SuSE Linux Openexchange Server (formerly SuSE Linux eMail
  Server). Standard, good open-source components (Postfix, Apache, Cyrus
  IMAP, OpenLDAP, OpenSSL) preconfigured to work well with one another,
  plus a couple of proprietary components: YaST2 for graphical
  administration, and SkyrixGreen for integrated scheduling and group
  discussions (shared folders). Client access from any OS, including but
  not limited to webmail. A full-functional trial version (lacking only
  "maintenance") is available for US $20 at http://www.suse.com/openexchange/slox_eval_form.ht ml
  [suse.com] . Sites are known to scale well to at least 1,000 users per
  site. The largest deployment yet known (March 2003) is 1,900 users.
• Bynari Insight Server, http://www.bynari.net/

  [bynari.net] . Server end is Linux-based. Intended as a plug-compatible
  replacement for MS-Exchange Server, based on POP3, IMPA, SMTP, and
  LDAP, but also with full support for all the special, proprietary
  MS-Exchange Server RPC-based protocols for group discussion,
  scheduling, contact management, task lists, etc., when used with
  MS-Outlook clients. Review: http://linuxjournal.com/article.php?sid=6734 [linuxjournal.com]

• Bynari InsightConnector, http://www.bynari.net/
  [bynari.net] . Extensions that load into MS-Outlook clients to let them
  perform MS-Exchange-type functions (scheduling, contact-management,
  public folders) without needing an MS-Exchange server, using only
  open-standard IMAP, SMTP, and LDAP servers, instead.
• Samsung Contact (formerly HP Openmail), http://samsungcontact.com/en/

  [samsungcontact.com] . Server end can be Linux-based (or Solaris/AIX).
  Based on SMTP, IMAP, POP3, LDAP. Supports proprietary protocols for
  e-mail, scheduling, etc. native to Samsung's Contact client (which is
  available on Linux and Win32). Webmail access. Implements Microsoft's
  (documented, for a change) MAPI protocol for scheduling, public
  folders, offline folders.

• Oracle Collaboration Suite, http://www.oracle.com/ip/deploy/cs/ [oracle.com] . Formerly Steltor CorporateTime, http://www.steltor.com/
  [steltor.com], until that firm's recent acquisition by Oracle. (That
  product is said to have emerged from Netscape Calendar.) Does IMAP,
  POP3, SMTP, E-mail, real-time conferences, voicemail, scheduling.
  Apparently implements all of the special, proprietary MS-Exchange
  Server RPC-based protocols for group discussion, scheduling, contact
  management, task lists, etc., when used with MS-Outlook clients. Uses
  Oracle 9i as its message store. Web site doesn't really make clear what
  OSes the server end runs on.
• CommuniGate Pro Messaging Server, http://www.stalker.com/CommuniGatePro/

  [stalker.com] . Server end runs on lots of Unixes including Linux, or
  on Win32, and quite a few other OSes. Does IMAP, POP3, SMTP, LDAP,
  scheduling, webmail. Can implement Implements Microsoft's (documented,
  for a change) MAPI protocol for access to its features from MS-Outlook.

• EasyGate Workgroup, http://download.easygate.n-h.com/
  [n-h.com] (Don't know much about this, yet. Appears to be a superset
  incorporating exchange4linux, about which see separate entry.)
• Kerio MailServer, http://www.kerio.com/kms_home.html
  [kerio.com] . IMAP, POP3, SMTP, antivirus/antispam, public mail
  folders, shared mail folders, webmail. Optional integration into MS
  Active Directory.
• SCOoffice Mail Server, formerly Caldera Volution Messaging
  Server: Postfix for the MTA, Cyrus for message storage, Horde/IMP/MySQL
  for webmail, and OpenSSL and pam-ldap for authentication. Lacks
  scheduling, but SCOoffice Mail Connector for Microsoft Outlook is
  available separately. http://www.sco.com/products/SCOoffice/mail/ [sco.com] http://www.linuxjournal.com/article.php?sid=5663 [linuxjournal.com]
• Joydesk, http://www.joydesk.com/ [joydesk.com] . Proprietary, uses Borland Interbase for data storage.
• Kroupware, upcoming, working in beta stage. Postfix for
  the MTA, Cyrus for the LDAP core, can be used with Outlook when a
  Bynari connector for Exchange is installed. Includes scheduling and
  tasklist modules, which establish new, open-source standards. Imagen's
  Pegasus Mail already supports the latter, as does the new KDE Kolab
  client.
  http://www.kroupware.org/ [kroupware.org]
  http://linuxjournal.com/article.php?sid=6734 [linuxjournal.com]
• Sherpath, http://www.sherpath.org/
  [sherpath.org] , beta stage. Client access via Web browser (only). 100%
  open source: Implemented with PHP4, Apache, MySQL in PHP + HTML +
  JavaScript. E-mail, scheduling, contact mgmt, task mgmt.
• OSER (Open Source Exchange Replacement, http://oser.sourceforge.net/
  [sourceforge.net] ) seems to be in very early development, and will use
  Qmail (proprietary, source-available), Courier IMAP, OpenLDAP,
  Samba-TNG, Jabber, Jical, Squirrel Mail, and so on, to provide e-mail
  and group discussions (public folders), IM, and scheduling.
• exchange4linux (including the older BILL Workgroup Server server-side codebase, http://www.billworkgroup.org/billworkgroup/home
  [billworkgroup.org]) from Neuberger & Hughes GmbH (Netherlands) is
  a suite of tools to serve mail, public folders, task lists, shared
  calendars, etc. to MS-Outlook (but not Outlook Express) clients
  operating in Corporate / Workgroup mode. Server code is open source;
  the BILL MAPI Service Provider for Outlook (which must run on the
  MS-Windows/Outlook client side) is proprietary. Web-based access, an
  IMAP gateway, and an LDAP export feature are under development.
  Supports a single-level global Outlook address book, ACLs on public
  folders (including Appointments, Contacts, Notes, etc.). Support for
  opening another user's private mailbox or calendar is under development.

Update (2010) : As someone mentioned in the comments, this article is *old*. You might want to have a look at this article, which mentions Zimbra, Open-Xchange, Citadel and Kolab as worthy Exchange replacements.