When I was in the Marines, I had a firearms instructor who taught me an important principle: You don't often need a pistol, but when you need one, you really need one—nothing else will do as a substitute. It's fair to say that the same often holds true for email archiving systems. Most companies who deploy them are trying to meet a particular set of legal or regulatory requirements. Because necessity is the mother of invention, an entire industry has sprung up to provide solutions for capturing, indexing, retaining, and retrieving messages for Exchange Server, IBM Lotus Notes, and other messaging systems. These solutions come in many flavors, including appliances, hosted services, and software that you install on your server.
Exchange doesn't offer any direct archiving capability. Instead, Microsoft has focused its efforts on providing a reliable journaling mechanism. "Journaling" refers to the activity of capturing individual messages in a form that an external archiving system can utilize. This sounds like it's no big deal, but there are actually a surprising number of ways in which a message can fail to be journaled. For example, it's critical to capture Bcc recipient information as part of the journaling process to keep from missing messages. Microsoft has steadily improved Exchange journaling, but they haven't added any direct archiving functionality . . . at least, not to Exchange. SharePoint, however, is another story.
In Microsoft Office SharePoint Server 2007 (MOSS), Microsoft added a number of records management features. Most of the time, when people talk about records management, they really mean two things: the ability to search documents and the ability to cause them to expire after a set period of time. From a messaging standpoint, the interesting thing about MOSS's Records Center functionality is its ability to store messages that have been journaled by an Exchange server and make them available for indexing and expiration.
However, there's a significant gotcha: When Exchange sends messages to SharePoint, the messages might contain content in Transport Neutral Encapsulation Format (TNEF), and SharePoint can't currently index those messages. This limitation means that MOSS doesn't make a very useful archiving target for email messages, although it might work fine for archiving other kinds of content.
Fortunately, a fix is on the way. One of the improvements scheduled for Exchange 2007 SP1 is a change to the Exchange journaling engine to prevent it from sending TNEF attachments. After you install SP1, you'll be able to use Exchange journaling to send messages to MOSS for retention and indexing. Until then, though, don't plan on using SharePoint as a long-term storage mechanism for your Exchange mail messages.