Hi Krish,
In order to send email from a different user, 2 permissions/ settings are required.
The user mentioned in the from address should have the setting checked under personal options “Allow other Microsoft Dynamics CRM users to send e-mail on your behalf” which appears under Options >> Email tab.
Also, the user triggering/sending that email should also have "Send Email as Another User" security permissions.
Do not that if you are sending it through real time workflow, and the workflow is set to execute as owner of the workflow then you need to set the "Send Email as Another User" security permissions for the owner of that workflow as well.
worldofitguy.com/.../user-does-not-have-send-as-privilege-for-crm-2015
Hope this helps.