Outlook Web App (OWA)
Outlook Enterprise uses a corporate Exchange Server. The Exchange Server offers each user a personal mailbox as well as shared public folders. Users can access their mailbox and the public folders with Microsoft Outlook on their desktop.The Exchange Server also allows Users to share information within a Group, say the Finance department.

Outlook Web App gives Users a simple way to access a their Exchange Server from any web browser. Many government agencies and businesses use Outlook Web App with an Exchange Server. For example, the County first responders use Outlook Web App on their Smart Phones and laptops. Outlook Web App is light and mobile.

The key idea here is the Exchange Server: it is the Server that publishes the calendars and replies with an Out Of Office message.

The data is stored on the Exchange Server, which is backed up regularly.

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Example of Microsoft Outlook Web App (OWA)