Removal of Site Mailboxes in SharePoint Online
Action Required by: March 1, 2017
Details: Beginning March 2017, new SharePoint Online organizations
will no longer have access to the Site Mailbox feature, existing
SharePoint Online organizations will no longer be able to create new
Site Mailboxes; however, any Site Mailboxes provisioned prior to March
2017 will continue to function. 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 are using the Site
Mailbox feature in SharePoint Online, you should consider
transitioning to Office 365 Groups, to support email-centric
This is quote from Office 365 Planned Service Changes for 2017 – Admin Action Required. So as stated, the Site Mailbox feature can no longer be activated with SharePoint Online.
The alternative as provided by Microsoft would be to move to Office 365 Groups. O365 Groups support email-centric collaborative scenarios, which should fulfill e.g. SharePoint's Site Mailbox functionality. Learn about Office 365 groups.
You can active Site Mailbox by going to Site settings > Site Features > Activate Site Mailbox.