It takes a special kind of maturity to disagree with someone without insulting, demeaning, or disrespecting them. If a person disagrees with you and the first thing they do is to insult or pull an ad hominem, you are dealing with an individual with a disgustingly low EQ and IQ.
"The greatest remedy for anger is delay."
— Seneca
Sit on the message for one night.
Sleep on the email for eight hours.
The reply you would have sent at 9 pm and the reply you will send at 6 am are not written by the same man.
Be thankful for the small achievements in your life.
Don't live another person's life.
Run your race. It is fulfilling.
Be patient. Your reward will come.