Just saw a notification in my Office 365 portal that site mailboxes are being removed from SharePoint online. Existing site mailboxes will continue to function for now, but after March 2017 you cannot create new site mailboxes.
It is recommended that you use Office 365 groups for collaboration instead. An Office 365 group behaves like a combination of a distribution group and shared mailbox combined with storage in SharePoint. It's a more complete collaboration solution, but you can use just the features that you want.
In September 2017, a process will begin to transition site mailboxes to Office 365 groups.
Here is a link with more info about Office 365 groups: