Documentation

Using tenant-wide settings your Farm (SharePoint) administrator can optionally allow only specific mailboxes to be available for setting up import profiles. This allow the administrator to prevent access to any mailbox that is not explicitly authorized for import purposes, for privacy and security reasons. Note that import profiles that were added before this feature became available won't be affected immediately, but you will no longer be able to re-save those profiles, if the email they use is not in the list configured by the administrator, assuming that they actually used this setting.