How can we Integrate them so that both (Manage Engine and Splunk ES Incident review) works in sync
Have a look at the REST API:
https://help.servicedeskplus.com/api/rest-api.html
although we can generate ticket using email, how do we do that using REST API
This likely requires you to engage with a Splunk partner with established Splunk development experience. It’s complicated and well outside scope of what someone can do via Splunk answers
