I want to send mails to user when his asset gets unpublished using the OOTB feature.
So, after I configure the Day CQ Mail Service and add email ID to the user details, is there anything else that I need to do? Or will the user automatically receive the email?
You can configure a workflow launcher to run on publish with the event type "node removed". In your workflow model you are going to check if the deleted node is a dam resource and if so, the next step is to configure an email step to send an email to your preferred user. The user and his email should exist on publish. If your users exist only on the author instance, you need a custom step with some kind of publish author notification mechanism or alternativ reverse replication.