How do you know? SMTP wasn't designed for reliable delivery. You don't know if your emails are being received, and even i they are, you don't know if they'll be received tomorrow.
Statistically. I communicate with a lot of people via email, and if I don't get an expected response to something of consequence I follow up - it's far more common the recipient simply missed it/didn't get around to it than it is to get "I never got it/I found it in my spam folder".
