An external list in SharePoint that contains customers, tasks, appointments, or posts can interact with with Microsoft Outlook 2013. Once connected, you can view, edit, and print the data using the familiar Outlook user interface If the data is contact information, such as customers, you can even call them, send them e-mail messages, and make meeting requests.
To achieve this function, we need to enable the site feature named Offline Synchronization for External List as follows.
a. Go to Site Settings -> Site Features
b. Activate Offline Synchronization for External List Feature.