I'm a 22-year-old college student soon to be entering my final semester of my undergraduate degree. For the second time this semester (and for entirely unrelated occasions), I've been contacted by former teachers via e-mail. Both of these teachers are figures from my elementary school, which runs from Kindergarten through 5th grade where I'm from. It is unlikely that I've interacted with either person since entering the 6th grade and certainly not within the past five years. Both cases have involved requests of me to return to my elementary school and speak with students about the academic and professional experiences I've had throughout college.
I'm perfectly comfortable with responding to these requests up but I'm unsure of how to address the e-mail. On one hand, every encounter I've ever had with either teacher occurred prior to my teenage years -- addressing them by anything other than "Ms." or "Mr." seems strange. On the other hand, I'm an adult and the power balance inherent in the teacher-student relationship we once had has dissolved, so it seems equally strange to maintain the formality.
I realize I am probably overthinking this but it's the second incident in recent times that I've faced this question. What is the correct way to address one of your former teachers in a casual setting? Is it improper to address a former teacher by their first name unless invited to, or is it more awkward to maintain the formality? My question was spurred by an e-mail, but I'm from a relatively small town and the chances of me encountering a former teacher while out and about back home are high, so I'm curious as to what the proper etiquette is in a more general sense.