With the release of Dynamics 365 Customer Engagement (9.0), older versions of TLS (1.0 and 1.1) connections will be blocked and only connections using TLS 1.2 will be supported. If you are connecting a Microsoft Dynamics 365 for Outlook client on version 8.x to a 9.x Dynamics 365 instance, an update will be required for the clients.

Please refer to the following articles for details:

Updates coming to Dynamics 365 Customer Engagement connection security

"There is a problem communicating with the Microsoft Dynamics 365 server" error occurs when configuring Microsoft Dynamics 365 for Outlook