How I respond when students use the title "Mister" (not "Doctor") to address me

I say, "Yes?"

An hour ago I received an email from a student who is still in high school. Apparently she is writing a research paper for her class, and she found an article of mine she wanted to use that was behind a paywall. She was emailing me to request a PDF of the article. "Dear Mr. Whitehead," her email began. And so on. It all seemed very formal and polite. 

I responded to her with the same respect and dignity she had allowed me, which was as much as I could muster.

It doesn't bother me when students mispronounce my name or use the wrong title or forget who I am. Most of my students call me "Mr. Whitehead." This succeeds in getting my attention, which I think is their goal in addressing me to begin with. I have never gotten the sense that students were saying "Mr." in an effort to insult me. Indeed, I always detect at least a little hint of deference. Most students, I have come to realize, are uncomfortable addressing their professors, and, consequently, get nervous. We do all sorts of mixed up things when we get nervous. 

[Note: a female colleague of mine once suggested that it was because she was a woman that her students call her "Mrs. ____ rather than Professor _____ or Dr. ____." She assumed that male faculty were given special treatment. That has not been my experience.]

I have also been called Mr. White, Whitehouse, Mr. Patrick, Mr. Whited, and Professor Stanislaus (I suspect this student had confused me with their economics professor).

I am only occasionally called "Dr. Whitehead" by students, even though that is how I introduce myself to them at the beginning of the semester, and how I refer to myself during class. 

Don't get me wrong. I am proud of my doctorate and my title (professor). But I realize that these don't make the same impression on my undergraduate students. Students in pursuit of a doctorate and other doctorate-holding persons, it seems, are the only ones who really care about the title. I'm fine with that.

-Mister Doctor Professor Patrick M Whitehead, PhD