This happens through Autodiscover, and Auto-mapping is enabled by default. Auto-mapping will automatically connect Outlook users to mailboxes that they have been granted mailbox permissions to. When you assign mailbox permissions, you have the option to enable or disable auto-mapping. The access granted through mailbox permissions is “Full Access”, meaning that the user can read, write, edit, create, delete, and so on.
Every folder within the mailbox, whether it be the Inbox, Calendar, or Contacts, allows the same level of access, when mailbox permissions are used. Mailbox permissions are used to grant access to an entire mailbox. The two approaches are suitable for different scenarios. When you are granting access for one user to access another mailbox, whether that be another user’s mailbox or a shared mailbox, you can configure the access using either mailbox permissions or mailbox folder permissions.
