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A Microsoft Exchange 2013 site mailbox is an Exchange 2013 feature that helps facilitate collaboration between SharePoint 2013 users.
Introduced in the Exchange Server 2013 release, site mailboxes give users access to both Exchange email and SharePoint documents through Microsoft Outlook 2013. Site mailboxes are tied to a corresponding SharePoint site, and site members can drag and drop documents as well as email messages into the site mailbox's Documents folder, which is located within their Outlook client. Site members also have the option to cc the Exchange 2013 site mailbox as a way to log information, conversations and documents.
To use Exchange 2013 site mailboxes, a company must have Outlook 2013, SharePoint 2013 and Exchange Server 2013 deployed.