Crm Plugin Not getting triggered

Hope the below link will be useful for you, but the version they used is CRM 2011 not Dynamics 365.

https://social.microsoft.com/Forums/en-US/389017d4-fad0-4af2-8722-749a80372805/crm-2011-online-how-to-convert-an-email-to-case?forum=crm

To Debug: https://community.dynamics.com/crm/f/117/t/192605

Here is the Out of the box feature for Dynamics 365, Creating cases from email using rules: https://www.microsoft.com/en-us/dynamics/crm-customer-center/automatically-create-a-case-from-an-email-customer-service.aspx