I think delivering the car with all his stuff inside and posting the keys and note through the door is such a classy move. Don't guild the lily by doing anything more than that.
Its clear, precise and deadly in its execution. Your cool calm approach and the deafening silence that will follow will really sting his ego. He thought he was in total control and will learn how wrong he was.
Keep the note very factual and precise. Don't mention you've seen the messages.
I would address the envelope to Mr First Name and Surname.
In the letter I would write:
I have instructed my solicitor ( insert name and firm) to file for divorce, he will be in touch to advise you of the next steps. I intend to push this through as quickly as possible. The girls and I are looking forward to getting on with our lives in a more positive environment.
All future communication should go through the following email address (insert) or my solicitor. I will not respond except through these channels.
Insert your name.
The more cold and professional you are the more it will sting.