Synchronization with Outlook is only available within the settable sync now as the MSDN saids.

Title : What fields can be synchronized with Outlook?
URL : https://docs.microsoft.com/en-us/dynamics365/customerengagement/on-premises/admin/what-fields-synchronized-outlook

If this function could be more flexible such as one-way sync only for all the fields, it will make Dynamics 365 (Online) more user friendly.