The Availability service retrieves free/busy information directly from the target mailbox for users on Exchange 2013, Exchange 2010, or Exchange 2007 and can be configured to retrieve free/busy information for users on earlier versions of Exchange. For topologies that have Exchange 2007, Exchange 2010, or Exchange 2013 mailboxes in which all clients are running Outlook 2007 or higher, the Availability service is used to retrieve free/busy information.

Outlook uses the Exchange Autodiscover service to obtain the URL of the Availability service. For more information about the Autodiscover service, see Autodiscover Service.

You can use the Exchange Management Shell to configure the Availability service. You can't use the Exchange Administration Center (EAC) to configure the Availability service.

The Availability service also provides access to automatic-reply messages that users send when they are out of the office or away for an extended period of time.

Information workers use the Automatic Replies feature (formerly known as Out of Office) in Outlook and Outlook Web App to alert others when they're unavailable to respond to email messages. This functionality makes it easier to set and manage automatic-reply messages for both information workers and administrators.

The Availability service is part of the Exchange 2013 programming interface. It's available as a web service to let developers write third-party tools for integration purposes.

Using Network Load Balancing (NLB) on your Client Access servers that are running the Availability service can improve performance and reliability for your users who rely on free/busy information. Outlook discovers the Availability service URL using the Autodiscover service. To use NLB with the Availability service, you must make changes to your configuration.

The internal URL is used from the intranet, and the external URL is used from the Internet. If you want to use the same URL for both internal and external traffic, make sure that DNS is correctly configured to route internal traffic directly to the internal URL. Also, make sure that the URL can be accessed both internally and externally. For the Autodiscover and Availability services to work, DNS must be configured so that mail.<domain name>.com and autodiscover.mail.<domain name>.com point to the virtual IP (VIP) of your load-balancing solution, where <domain name> is the name of your domain.

Note:
For more information, see Network Load Balancing Technical Reference and Network Load Balancing Clusters. You can also search for third-party load-balancing software websites.

The following table lists the different methods used to retrieve free/busy information in different single-forest topologies.