Any idea on how to send email notification once project milestone is checked on sales order record ?

This can be done with a workflow using the Send Email action.

@Raghu yes you could use a saved search to define your criteria, then use that saved search to trigger a scheduled workflow with the Send email action in it. Or if you need it to be more realtime you could use a trigger from the Sales Order (like after submit) to send an email as well. This is a great use of workflows to accomplish this.

Thank you guys👍