It is always nice to get emails (especially from out of the blue) that validate not only that someone you have taught is doing well but that you had some small part in their learning and success:
“It’s been a while, but I hope this email finds you well in your second year at Drew University! I just finished writing my final English paper of my college undergraduate career and I wanted to take this time to send you an email thanking you for the huge impact you’ve had on my life. I had to write a self-reflection paper and through writing that paper I was once again was reminded of how much I look up to you. You helped me improve my writing immensely, and also helped shape the way I view and move through the world. I know this is horribly cheesy, but you helped me not just become a better student, but a better person, as well. This whole quarter I found myself wishing you still taught at UW, it would have been awesome to take my senior capstone course with you!”