Your existing on-premises Exchange 2003 server supports allowing your users to access their mailbox using Outlook Web App. Outlook Web App provides users with a browser-based e-mail experience that is similar to the experience when using the Outlook 2003 mail client.

To launch Outlook Web App from outside the corporate network, users typically type the externally accessible FQDN of the mailbox server followed by the Outlook Web App virtual directory name, such as https://mail.contoso.com/exchange, in a Web browser and then provide their network credentials when prompted. Users can also typically launch Outlook Web App from the internal network by typing the internal server name followed by the Outlook Web App virtual directory name, such as https://mailserver/exchange, in a Web browser. Network user credentials are also required when launching Outlook Web App from the internal network.

In a hybrid deployment, you are typically moving existing user mailboxes from your on-premises Exchange 2003 mailbox server to the cloud-based organization in phases. Maintaining the ability to access mailboxes using a Web browser for users during mailbox migration is often very important.

Outlook Web App, formerly known as Outlook Web Access, is available for user mailboxes that are moved to the cloud-based organization and provides a similar Web-based messaging experience. Instead of an Outlook 2003-based user experience, the cloud-based organization offers a messaging experience modeled on Outlook 2010. Many of the features are similar and several new features are also included. However, users will need to use the cloud-based organization Outlook Web App address to launch Outlook Web App for mailboxes located in the cloud-based organization. Although you can choose to keep your existing Outlook Web App configuration in place during a hybrid deployment, we recommend that you configure the hybrid server to handle Outlook Web App requests and use a single URL to access user mailboxes.

Using the installed Client Access server role, you can configure the hybrid server to take one of the following actions depending on the location of the user's mailbox:

• Mailbox on the on-premises server   For mailboxes located on the on-premises Exchange 2003 mailbox server, Outlook Web App requests made to the hybrid server are redirected directly to the on-premises mailbox server. The on-premises Exchange 2003 mailbox server prompts the user for network credentials.
  
  
• Mailbox in the cloud   For mailboxes located in the cloud-based organization, Outlook Web App requests made to the hybrid server are not automatically redirected to the cloud-based organization. Instead, the hybrid server displays a Web page that provides users with a link to the Outlook Web App endpoint for the cloud-based organization. Users also have the option to save the URL as a favorite in their Web browser.
  
  

Configuring the hybrid server to take these actions requires the following process:

1. Configure the Outlook Web App virtual directory on the hybrid server to redirect Outlook Web App requests to the on-premises server for mailboxes located on-premises.
   
   
2. Configure the organization relationship on the hybrid server to display the cloud-based Outlook Web App URL for user mailboxes located in the cloud-based organization.
   
   
3. Update the existing DNS record to direct Outlook Web App requests to the hybrid server.