Consider the following scenario:
You're using an Exchange Online user role policy through which the MyTeamMailboxes role is disabled.
You browse to a SharePoint Online site, click add an app, and then click Site Mailbox. Or, you browse to Site settings, click Manage site features, and then click Activate for the Site Mailbox.
When you browse to the site mailbox in this situation, you receive the following message:
Sorry! Access denied :(
You don't have permission to open this page. If you're a new user or were recently assigned credentials, please wait 15 minutes and try again.
You're still signed in. If you want to sign out, use the link below.
This is a known issue. Although user site mailbox creation was disabled in Exchange Online through a user role policy, the option to create a site mailbox still appears in SharePoint Online. Therefore, when you try to use the UI features that rely on the site mailbox functionality, you receive the error message.
In this scenario, Global Administrators for an organization in Office 365 can still create site mailboxes.
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