I need to know how do I get to automatically send notifications to a user who
is available in the company's outlook address book whenever a change in a
specific excel sheet has been made.

To be more clear, I have a report on an excel sheet that is devided to 12
parts (the number of the agents in our call center). The report contains
parts related to agents' performances. What I need is to have an auto message
sent through outlook to each use when a cell (in any agent's part) has been
modified with a data stating that their performance is below the target.