Microsoft Exchange Server public folders have been around since the introduction of Exchange back in 1996. Since that time, they’ve been used for a wide variety of purposes, not all of which were anticipated by Microsoft’s designers.

The basic concept behind public folders is simple: You can create a folder that's accessible to multiple users at the same time and put various kinds of data (including email messages—known as posts—calendar items, or contacts) into the folders. By setting the ACL, you can control which users can create, read, modify, and delete information in each folder; by creating replicas of a folder on multiple servers, you can make the data in the folder available throughout an organization without having a single point of access (or a single point of failure).

Exchange uses public folders internally for two key purposes. First, a special set of public folders holds the system-generated Offline Address Book (OAB). The format of the Exchange OAB has changed several times over the years, so each version is stored in a separate folder. Outlook clients connect to the appropriate folder and download the correct version of the OAB. Second, the system SCHEDULE+ FREE BUSY folder stores information from users' calendars about when they are free or busy. This information is stored as a bitmap for each user. You can add replicas of the free/busy folder to provide local access in different sites, but the replication process might introduce latency.

There are also several common external uses for public folders.
— You can mail-enable a public folder so that email sent to its address is automatically posted to the folder. I use a public folder to receive reader email about my columns, for example, because it's a good way to collect email sent to an address and make it available to a group. Public folders provide per-user read/unread status data, too.
— You can store documents in public folders in two ways: documents can be attached to posts (in which case they look like regular message attachments) or they can be stored directly in folders (in which case they’re called "freedocs"). I know of one large Exchange site that has more than 250GB of document data stored in one public folder as a big, replicated file share.
— Applications can store data in public folders. Several workflow tools on the market route workflow items by moving them between public folders.
When I say the preceding uses are "common," though, bear in mind that customer data gathered through surveys at TechEd shows that only about half of Exchange customers use public folders.

Microsoft has been positioning its SharePoint products (Microsoft Windows SharePoint Services, a no-cost download; and Microsoft SharePoint Portal Server 2003, a larger-scale retail product with more capabilities) as its primary collaboration platform. As part of that positioning, Microsoft has said that new collaboration applications should target SharePoint rather than Exchange for data storage. That raises the question of how you get existing public folder data—which often forms the backbone of collaboration processes, if not applications—from Exchange into SharePoint. The answers, which I’ll share in next week’s column, might surprise you.