Server Side Synchronization in Dynamics CRM Online - Approve Email address

When using server-side synchronization in Dynamics CRM Online and if email address approval is enabled the email address must be approved by a Dynamics CRM user who has the approve email address privilege (usually a System Administrator) AND is an Office 365 Global Administrator.

Reference: https://blogs.msdn.microsoft.com/emeadcrmsupport/2014/09/16/dynamics-crm-2013-server-side-sync-email-approval/