Site Mailboxes were the face of Microsoft collaboration at one time. But that’s long in the past and it’s time for these archaic mailboxes to be dispatched. Microsoft will retire them from Office 365 in April 2021, probably two years after they passed their best-by date. I tried to clean up my tenant and failed utterly, so I’m leaving the mess for Microsoft to sort out.
https://office365itpros.com/2020/10/23/site-mailboxes-finally-retiring-april-2021/
