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 was hampered by the requirement to deploy SharePoint, and the complex integration steps to get Exchange and SharePoint to work together.
Although Office 365 makes integration between different workloads much simpler, because it's all controlled by Microsoft, Site Mailboxes haven't seen widespread adoption in the cloud. There's also been little to no further development of the feature, with all efforts going into Office 365 Groups instead.
In the Message Center notification, Microsoft wrote:
In September 2017, we will have a process in place allowing you to transition your existing Site Mailbox site to an Office 365 Group. We will communicate again in June 2017, via Message Center, with more details.
If you're using Site Mailboxes today (anybody?) you've got some migration work to look forward to. Otherwise, if you're looking for “email-centric collaboration”, Office 365 Groups are the way to go.