Sharing free/busy information between your on-premises and cloud-based organizations is one of the primary feature benefits of a hybrid deployment. However, there are some feature limitations for on-premises Exchange 2003 Outlook Web Access users and your Office 365 Exchange Online organization users.

Let's look at the following example scenario:

• On-premises Exchange 2003 organization - All on-premises mailboxes are hosted on an Exchange 2003 Mailbox server and you have used the Exchange Server Deployment Assistant to configure a hybrid deployment (formerly "rich coexistence") with Office 365 Exchange Online. An Exchange 2010 SP1 CAS/Hub Transport server has been added to your on-premises Exchange organization and you are redirecting user Outlook Web Access requests to the appropriate mailbox location (the Exchange 2003 Mailbox server for on-premises mailboxes and to Exchange Online for cloud-based mailboxes).
• Cloud-based Exchange Online organization - You have created an Exchange Online organization, added your shared SMTP domain, configured Exchange Online for a hybrid deployment and migrated some of your on-premises user mailboxes to Exchange Online.

Everything seems to be working OK - when your on-premises users are using the Outlook client to connect to Exchange, they can access cloud-based user mailboxes and basic free/busy information just fine. Cloud-based users can also retrieve basic free/busy information for on-premises user mailboxes. Everyone, especially the Exchange administrator, is happy.

However, you notice that when on-premises users use Outlook Web Access to access Exchange 2003 mailboxes, they cannot retrieve free/busy information for cloud-based user mailboxes. Cloud-based users can still retrieve free/busy information as before. Suddenly, things aren't as rosy and happy for the Exchange administrator. What has happened and what can be done to allow on-premises Outlook Web Access users access free/busy information for cloud-based user mailboxes?

The answer is that on-premises users in an Exchange 2003 organization MUST use the Outlook client in order to retrieve free/busy information for cloud-based user mailboxes.

The technical reason why using Outlook Web Access causes this behavior is that Outlook Web Access connections from Exchange 2003 can’t make WebDAV (Web-based Distributed Authoring and Versioning) connections to a free/busy system folder to retrieve the free/busy information for cloud-based users. When on-premises users are migrated to the cloud-based organization, their LegacyDN attribute is changed to /EX:/o=<name of your organization>/ou=External (FYDIBOHF25SPDLT). Since Exchange 2010 does not support WebDAV connections, the Exchange 2003 server can't connect to External (FYDIBOHF25SPDLT) on the on-premises hybrid server for Outlook Web Access requests. Outlook clients do not experience this limitation because they use MAPI instead of WebDAV when connecting to External (FYDIBOHF25SPDLT).

For more detailed information on configuring a hybrid deployment and free/busy sharing, check out the following resources:

• Exchange Server Deployment Assistant
• Exchange Online Hybrid Deployment and Migration with Office 365 -- A Planning Primer