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A Comparison of Enterprise Mail Servers - Open Source and Otherwise


While open source mail servers have always dominated the mail transfer market, enterprises expect much more than simply mail transfer from their mail servers. For instance, enterprise mail servers need to provide features such as effective SPAM and virus protection, shared calenders and contacts, and seamless integration with a wide variety of mail clients and email capable devices. This comparison will briefly describe the evolution of open source mail servers and then examine some of the contenders before focusing on the primary open source candidates for enterprise deployment.

The mail servers discussed in this comparison:


Traditionally, an enterprise required two types of mail servers. An MTA, also known as an SMTP server, was used to transfer mail between organizations over the Internet. Since the introduction of Sendmail in the early 1980s, open source has dominated the MTA market, with a recent survey showing that the open source MTAs Sendmail, Postfix, Qmail, and Exim represent 74% marketshare. The second type of server, known as a messaging or groupware server, was used to provide additional intra-organizational features such as shared calenders and contacts. The first messaging servers used proprietary protocols--in other words, they did not support SMTP. As enterprises became more Internet-connected, the vendors of messaging servers added SMTP connectors to provide MTA functionality. IBM Lotus Notes, Microsoft Exchange, and Novell Groupwise are examples of enterprise-grade mail servers that provide both MTA and messaging functionality.

Over the years, open source components were created to provide messaging, anti-virus, and spam protection features. Many of these components have matured and, when stacked to create a complete solution, can provide most of the features enterprises have come to expect from their proprietary mail solutions. However, creating a "roll your own" messaging solution can quickly erode the savings enterprises get by no longer paying licensing fees to a traditional proprietary vendor. It takes a significant amount of time and talent to research which components provide the required features; integrate, test and support those components into a custom solution, and, if necessary, to develop any features not provided by the open source components. Fortunately, a new generation of open source enterprise mail server has emerged to provide vendor-supported, pre-integrated, and customizable solutions for the enterprise.


All of the solutions we examine provide:


    • feature-rich messaging for a variety of mail clients

    • offer a number of offerings which cater to an enterprise's size and budget

    • and provide a network of integration suppliers, support offerings, and third-party add-ons

Each provides an open source "community" edition, developer APIs, and some means of community support such as forums, wiki, and documentation. Two vendors, Open-Xchange and Zimbra, use primarily open source technologies within their solution.

Primary Candidates

Three commercial open source vendors stand out as potential enterprise mail solutions because they provide and support the features set out at the beginning of this comparison. These candidates are:


    • Open-Xchange

    • Zimbra

    • Scalix


Open-Xchange, a privately-held company headquartered in Tarrytown, N.Y., released its software under the GPL in August, 2004. Open-Xchange offers several editions: a free community edition for enterprises with experienced Linux staff, a commercial server edition for medium to large enterprises, a hosting edition for SaaS providers, and a turnkey appliance for small to medium enterprises with a limited IT department. Support options range from the freely available Oxpedia to various levels of service level agreements. Price lists are available by checking the webpage for each edition and support level. There are pricing points for government, academic, and the enterprise.


Open-Xchange provides a rich email and collaboration environment, document sharing, anti-virus and anti-spam, automatic updates, and backup/recovery. It is designed to interoperate with Outlook and existing Microsoft Exchange clients. OxTender for Mac OSX, currently in beta, allows Mac users to access email, appointments, documents, contacts and tasks through standard Mac applications like Mail, iCal and Address Book. Open-Xchange also offers an easy-to-use graphical and skinnable web interface for administration tasks.


Open-Xchange is comprised of several mature open source components, including: the Apache webserver, Tomcat servlet engine, MySQL database, OpenLDAP directory server, SpamAssassin, ClamAV for antivirus, and the Dovecot, Cyrus, and Courier mail servers. It supports many standards, protocols, and extensions including: AJAX, WebDAV, SOAP, Java, OSGi, and SyncML.





Open-Xchange provides automatic synchronization with Outlook and synchronization with any SyncML 1.1 compliant device.

A migration tool is available to move email messages, calendar entries, tasks, contacts, user groups, and mailboxes from Microsoft Exchange to the Open-Xchange Server. Partner solutions are also available to extend functionality with such features as process and project management, data recovery, and unified messaging with VoIP.

Netvibes UWA enables integration of web applications such as RSS feeds, podcasts, video channels,, and SAP Business One. The OSGi framework support enables easy integration of third-party applications, such as CRM and ERP. Moreover, updates of all OSGi modules can be installed with almost no downtime.


From an implementation point of view, all versions of Open-Xchange (except for the turnkey appliance) require knowledge of Linux mail systems, Apache, MySQL 5.x, and command line administration of Linux systems. Integration of add-ons and appliances may require experience in XML, OSGi, or UWA. OXpedia provides free installation and developer guides and partners are available to provide installation services for a fee.

Once the solution is in-place, users can use the webmail interface or their existing Outlook or Mac OSX client to perform their usual enterprise email tasks. Administrators will still want to be familiar with Linux command line utilities as the administrative GUI is a work in progress. Small enterprises with little or no IT staff with Linux experience should consider the appliance and may find the administrative GUI complete for their needs.


User Interface

Administrative Interface


Zimbra, a Yahoo! company, releases its source and binary code offerings under the YPL. It offers five on-site editions as well as a hosted edition which can be hosted either on Yahoo! servers or through a Zimbra hosting provider. Pricing for the on-site editions is available online. Zimbra offers a variety of support options ranging from open source forums and documentation to several levels of support level agreements.


Zimbra provides a rich email and collaboration environment, document sharing, anti-virus and anti-spam, automatic updates, and backup/recovery. It is designed to interoperate with Outlook and Mac OSX Entourage users. Its GUI administrative interface makes it easy to manage service classes, create and edit new accounts, create and edit distribution lists, manage email queues, view and manage systems status, and start system backups.


Zimbra is comprised of several mature open source components, including: Apache Tomcat, Postfix, MySQL, OpenLDAP, Lucene, ClamAV, SpamAssassin, and OpenLDAP. It also supports many standards, protocols, and extensions including: SMTP, LMTP, SOAP, XML, IMAP, POP, iCal, CalDAV, and AJAX.

Zimbra provides a complete set of services to plan and execute customer migrations from Exchange, Domino, GroupWise, or other legacy email systems. Documentation and scripts are made available for migrating existing user accounts.


Interoperability with Microsoft Exchange 2003 and integrated instant messaging are currently in beta. Connectors are available to support Microsoft Outlook and Entourage on Mac OS X 10.4.

The Mobile Web Client is interoperable with all devices with HTML capable browsers, including the Apple iPhone. Mobile Edition provides sync to any java enabled device as well as native OTA synchronization and push email to iPhone, Windows, Palm, and Symbian OS devices. Zimbra Mobile Connector for BES provides native OTA synchronization withl BlackBerry devices.

Zimlets provide integration with third party information systems such as unified messaging with asterisk.


Zimbra provides demos and screenshots of its user and administrative interface. The administrative interface is quite complete, making it useful for most tasks. The Zimbra Gallery allows administrators to easily extend their users' experience using zimlets, themes, and extras. The hosted solutions provide less customization but allow enterprises with limited IT resources to still benefit from an affordable and collaborative environment.


User Interface

Administrative Interface


Scalix was acquired by Xandros Inc. in 2007. It offers several editions: an enterprise edition, small business edition, hosting edition, and community edition (free for up to 10 premium users). Each edition is under a separate Scalix license. Scalix offers a variety of support options ranging from the open source wiki and documentation to several levels of support level agreements. Pricing requires contact with a sales representative.


Scalix offers replicated multi-server support with centralized administration from a single Management Console. It supports multiple independent mail server instances on a single server, each with its own independent user set, domain name, service processes and message store which is useful for a central group that manages IT for legally separate entities or or for hosting solutions. Scalix provides clustering, failover, and load balancing. Recovery folders allow administrators to recover emails on a per-user basis. Warious storage architectures, including any DAS and Fibre Channel or iSCSI, are supported. Connectors are provided for Microsoft Outlook and Novell Evolution clients.


Scalix is built upon OpenMail technology licensed from Hewlett Packard. The Scalix Collaboration Platform supports many standards, protocols, and extensions including: SOAP, REST, CalDAV, and AJAX. The Scalix ecosystem provides a list of certified ecosystem components (applications). Spam and virus protection are provided by add-on modules from Commtouch.


Scalix ActiveSync provides calendar and contact synchronization with a wide variety of devices, ranging from Apple's iPhone over Symbian-based devices from Nokia and Sony-Ericsson to PDAs and SmartPhones running Windows Mobile 2005 and higher. A separate product, NotifyLink Enterprise Mobility Solution for Scalix, provides Blackberry support.

Scalix and partner migration tools are available to migrate mailboxes, messages, public folders, calendars, and directory data from Exchange and other legacy systems. Scalix provides transparent server-to-server co-existence with Exchange for enterprises who need to maintain a mixed environment or migrate on a phased schedule.


The administrative interface allows for management of users, groups, resources, mailbox usage, and basic server settings. Developers should note that the source code is licensed under the SPL, a modified MPL license.


User Interface

Administrative Interface


Enterprise mail servers are feature-rich--in other words, they are complex and expensive. When researching a mail solution, consider where you want to spend your money: on licensing, vendor support, in-house configuration, or in-house development. Also consider which features are business requisites. Does the solution provide all the features needed? If not, what are the associated costs of developing or integrating third-party modules?

Each of the solutions discussed offer interoperability with most clients and devices. They provide either out-of-the-box support or APIs to allow integration with other applications such as instant messaging or VOiP. Enterprises should research each product's fine print to determine the specific supported versions of clients and devices as well as the amount of work required for application integration.

Final Thoughts

This comparison did not discuss network architecture from a security perspective. Some enterprises require a separation between their MTA (Internet facing) and messaging (intranet facing) servers. We also did not discuss the data security or privacy implications of using hosted services.

This work is licensed under a Creative Commons Attribution 3.0 Unported License
Creative Commons License.

This work is licensed under a Creative Commons Attribution 3.0 Unported License
Creative Commons License.


Posted @ Saturday, August 18, 2012 4:58 AM by tikaram_magar
Just wanted to note that Expanding Communication by Maxwell Worthington gives a super easy way to install, configure, and manage SOGO, postfix, dovecot, etc. The system is one of the most feature rich over source distributions. 
Posted @ Sunday, September 08, 2013 9:42 PM by mitchell klein
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