You’re lying to yourself. You got mugged. You even said it yourself - your customers started receiving emails from his other company! The writing is on the wall.

Do you have hard proof of the email your customer received?

Probably. Also remember that if posters space is specific or niche, clients usually look for other software to help them. The emails from the competitors are not a smoking gun proof. There is no way to prove or disprove it and only the investor knows what's up