Microsoft is notifying Office 365 customers that access to Site Mailboxes is being removed from SharePoint Online, and no more Site Mailboxes can be created from March 2017 onward. The functionality of existing Site Mailboxes in a tenant will continue for the time being.
Site Mailboxes originally appeared as a feature of Exchange Server 2013. Positioned as a collaboration tool for teams, Site Mailboxes stitched together an Exchange mailbox with a SharePoint site to allow users to collect email conversations in a single location, as well as access shared documents from both Outlook and SharePoint. On-premises adoption of Site Mailboxes