Understanding and Replacing Microsoft Exchange
When you look at Exchange and study its components, you find they number only four. The first is an information store or message store. The store holds individual user messages and has an access control list (ACL) engine associated with them. Similar to RFC-compliant IMAP servers, namespace differs according to whether the stores belong to individual users or whether the folders are public. Microsoft uses an Access database for storing message stores. The limitation of Microsoft's Jet Engine technology and the Access MDB file prevents vertical scalability.
Secondly, Exchange has a directory. Microsoft structured their Exchange directory with object classes and attributes. The Exchange directory structure resembles the RFC-compliant LDAP protocol. However, Microsoft added Object Classes and changed the attribute names within those and other classes.
Next, Exchange has a mail transfer agent or MTA. Microsoft's MTA appears similar to the MTA used in an earlier product called Microsoft Mail 3.5. The Microsoft Mail MTA requires connectors or gateways, which rewrite their proprietary mail headers to those that comply with foreign systems, such as Lotus Notes, X-400 and RFC 822 internet mail standards. Unlike sendmail and similar internet MTAs, Exchange's MTA lacks configuration options.
Finally, Exchange has a component called a system attendant. The attendant handles every action taken within Exchange, from sending and receiving e-mail to filling requests for addresses from the Exchange directory. In many ways the system attendant resembles an attempt to provide interprocess communication (IPC), which Microsoft's operating systems lack.
Our Linux server-side solution included similar components to those found in Exchange. The first is the Cyrus IMAP message store. Cyrus stores hold individual user messages and have an ACL engine associated with them. Namespace differs according to whether the stores belong to individual users or whether the folders are public. Cyrus uses the Berkeley Database from Sleepycat Software. Where Microsoft's Jet Engine and Access database technology prevents scaling, Berkeley DB's high performance and scalability support thousands of simultaneous users working on databases as large as 256 terabytes.
Secondly, Linux has a directory. While Microsoft structured their Exchange directory to resemble the Lightweight Directory Access Protocol (LDAP), the Linux solution uses OpenLDAP software, an open-source implementation of LDAP. To accommodate Outlook clients, we added the Exchange object classes and their noncompliant attribute names. We indexed the Microsoft-based distinguished names and created a high-performance global address list.
Like Exchange, the Linux solution has an MTA that can be managed and configured internally and doesn't need external connectors. The University of Cambridge developed the Linux MTA we use, called Exim. Exim has numerous configuration options, including file lookups, local delivery and regular expression support. In the context of the Linux MTA, users provide regular expressions to filter content coming in and going out.
In the “Exchange Replacement HOWTO”, Johnson and Mead leave the tasks of adding server-side messaging and the administrative console to the next generation of Linux developers. In this article, we explain how one could transform Exchange transports and message stores. We accomplish this in two steps. First, we capture Outlook messages and decode their TNEF objects. Secondly, we use the Exchange client extension architecture to add IMAP functionality to Outlook in its Corporate Workgroup mode.
These two steps can allow a programmer or a seasoned administrator to create an alternative service provider for Outlook and serve a number of conventional mail clients. Linux mail servers do not discriminate based on the platform one uses. One can use Netscape Mail, Outlook Express, Ximian Evolution, mutt or Pine, to mention a few of the available MUA.
Highly scalable Linux components, such as Cyrus IMAP, OpenLDAP and Exim, can replace dozens of Exchange servers on a single Intel platform. The layers of interfaces and outdated DCE components used by Exchange do not hinder Linux. With Linux on the zSeries mainframe, we can replace hundreds of Exchange servers.
If you're looking for a graphical administrative console, projects such as PHP Cyrus tools, cyrus_imap-sql, Webmin and Replex can make administration of the server a simple task.
In general, few people would consider replacing Exchange with Linux an easy task. In spite of that, our development team proved that it could be done. Hopefully, we have taken much of the mystery and intimidation out of the Exchange server.