With Exchange Server 2007 as your messaging platform, secure access to more consistent and up-to-date free/busy information is now possible.

Unlike previous versions of Exchange, Exchange Server 2007 does not have to publish free/busy information into public folders if all attendees have Exchange 2007 mailboxes and are using Outlook 2007 or 2007 Outlook Web Access. In this “native” 2007 messaging environment Exchange Server 2007 makes free/busy information available in real-time directly from the attendee’s mailbox. This means you no longer have to deal with replication delays and access latency commonly associated with public folders.

All this is made possible by the Availability Service, which supportes clients like Outlook 2007 and 2007 Outlook Web Access, via the Auto-discover service.

The Availability Service is a Web Service deployed on the Client Access Server (CAS) role of Exchange Server 2007 along with the Auto-discover service. (For more information on CAS see http://technet.microsoft.com/en-us/library/bb125134.aspx)

So how does this all work?

First, the client will make a connection to the CAS.
- If Outlook 2007, the CAS will be determined via the Auto-Discover configuration using the Availability URL.
- If the target mailbox is in another AD site, the source CAS will make an HTTPS connection to the target CAS server. The target CAS will obtain the free/busy info by communicating over MAPI to the mailbox server and then send it back to the source CAS.
- If the target mailbox is in the same AD site then the CAS will communicate to the mailbox server via MAPI and obtain the free/busy info. The source CAS will then send the data back to the client.

For backwards compatibility, Exchange Server 2007 will still publish free/busy information to public folders in mixed messaging environments and provide other access methods. For example,
1. When the e-mail client requesting free/busy information is Outlook 2003 and the user using this client as well as the target attendee have Exchange 2007 mailboxes, free/busy information will be published in local public folders.
2. When the e-mail client requesting free/busy information is Outlook 2007, the user using this client has an Exchange 2007 mailbox, and the target attendee has an Exchange 2003 mailbox, the availability service will make HTTP connections to public virtual directory of the Exchange 2003 mailbox.

For more information about Exchange Server 2007 please visit the following link: http://technet.microsoft.com/en-us/library/aa996018.aspx

Ian