In SP site owners can activate the site feature MyMailbox. This create an Exchange mailbox within the site with the email address SMO-<SiteName>@tenantname.onmicrosoft.com. If our Exchange Admins turn of the permission to create a InMailbox in exchange the site owner can still activate the feature, see the mailbox created but upon accessing receive an 403 error. it would be much cooler if the actîvation of the feature is geyed out in someone their SP site.