Site mailboxes allow users to work together naturally – while compliance is applied behind the scenes.
A site mailbox brings Exchange emails and SharePoint documents together. For users, a site mailbox serves as a central filing cabinet, providing a place to file project emails and documents that can be only accessed and edited by site members.
When one project member files mail or documents using the site mailbox, any project member can then access the content. Site mailboxes are surfaced in Outlook 2013 and give users easy access to the email and documents for the projects they care about. Additionally, the same set of content can be accessed directly from the SharePoint site. (Site mailboxes don’t appear alongside users’ personal mailboxes in Outlook Web App.)
Under the covers of a site mailbox, the content is kept where it belongs. Exchange stores the email, providing users with the same message view for email conversations that they use every day for their own mailboxes. Meanwhile, SharePoint stores the documents, bringing document coauthoring and versioning to the table. Exchange synchronizes just enough metadata from SharePoint to create the document view in Outlook (e.g. document title, last modified date, last modified author, size).
Compliance policies are applied at the site mailbox level and are transparent to the users – thus adding value behind the scenes while preserving users’ productivity.
Source: Exchange Team - Site Mailboxes