The invitation to interview emails are written and signed by a single person (a director of admissions, for example), but the interview invitation email is usually sent from the school's primary admissions email address, something like
[email protected]. So if you just hit reply, that person will probably never see your thank your email.
Is it appropriate to find the person's (the person who signed the email) email address on the school's website and then send them a short thank you email for the invitation to interview?