Okay, so here's my (last?) update on the email situation:
After talking to Ms. Dockery at the Office of Admissions, I was able to have an email sent from her personal Duke email address, to which I replied with my acceptance of the waitlist position. I might personally recommend waitlisted applicants call and ask for verification that their response to the waitlist offer has been received.
With regard to the email addresses in question, I called Duke OIT, but they were unable to verify for me whether a given email address is a valid receiving mailbox. However,
[email protected] is the listed contact email for admissions, and is definitely real, since I received the post-interview email from this address.
[email protected] is not listed anywhere online, but definitely exists, at least as an outgoing mailbox, since the waitlist offer came from this address.
[email protected] is not listed anywhere online, yields an invalid address bounceback from the server, and is invalid according to multiple email verification services online (the above two address are unverifiable due to server settings; one would expect the same result if this address existed).
Thus, I conclude that
[email protected] in fact does not exist, and the Office of Admissions is mistaken in directing applicants to send email here. I have no idea how to prove this to them, however, short of telling them to send an email from their outgoing box to the address.