The Friday email
Late on Friday, the customer's operations director emails you, copying the sponsor: “Your team's gross negligence has delayed our production.” You know the delay was caused mainly by the customer's own late utilities, and you have the records.
Your first instinct: reply to everyone, attach the evidence, prove you are right.
You might win the argument and lose the customer. Emotional intelligence is the skill of choosing the response that works, not the one that feels good right now.