I was emailing someone in admissions and the admissions officer replied back saying I shouldn't be addressing them by their first name. This individual wants me to refer them as Mr/Mrs.. I've been working in a professional environment for a long time and thought it was always acceptable to address someone by their first name (maybe except professors/doctors). I felt like I offended this individual. My question is do I keep continue replying to other admission officer's emails by their first name or should I use Mr./Mrs. from here on out?