How to send an email after a specific node field gets updated by an admin? I need to send an email notification to the node editor (reviewer/moderator) when an admin changes a (custom) field.

I've added the list of editors as a field in node form.

The nodes will be created by users and unpublished by default.

The nodes will be assigned to editor for review by admin, the editor should be able to view records assigned to the editor.

When admin assigns a node to editor, the editor should be notified by email. 

How can I notify editors when they are assigned a node by admin?

Rules requires data comparison, conditions to trigger action. But I need to trigger action when a particular field gets changed.

I would like to use Drupal hooks like hook_node_update, by that way we can get the email id of the editor (in my scenario) and notify the editor.

Programmatically where and how I use hook_node_update() to achieve this? Or can the Rules module be used? If so please explain in detail how to create a rule for this.
Check notification email link
my point is enough to post in comment box so i am posted here