This email doesn't need a reply.
You should simply follow the advice in the email.
The email is implying that "they" (probably some customer) were cc'ed on too many emails. Possibly these were emails related to the customer e.g. technical details regarding their project, but not really important to them.
They don't want to receive those emails because it's a cost for them. They have to either thoroughly read through emails which are not important or they will learn to ignore them and risk missing some important detail in one of them.
So it's a reasonable expectation that they only get emails which are essential, i.e. contain either important information or require a reply from them.
The person who wrote an email to you claims that she/he has a good judgement which email is or isn't essential for this customer and is offering his help if you'd have doubts in the future.
NOTE: It's possible that the emails haven't upset anyone yet and the person who wrote you the email is trying to reduce email tsunami before it becomes a problem.