I respectfully disagree.
You can always find the address for your interviewer. Worse come to worse, mail the letter to the school with your interviewer's name at the top. That person will get it eventually.
I think perhaps younger generations are OK with emails, and I guess I'm not
that old
, but I really think the older crowd do appreciate a written letter more.
Around Christmas time, would you send an email 'thank you' card to your boss after he/she gave you a gift card, for example? I sure hope you wouldn't! If you get into the school of your choice, that would be the best 'gift' ever, right? That interviewer might have gone out on a limb to represent you at the admissions meetings. They put a lot of time into those committees. So just take 5 minutes to properly express your gratitude.
To the OP: I would strongly encourage you find the address of the school and send a real thank you letter. It takes hardly any time and shows a lot more 'thanks' on your part.