If it is faculty in College or University, it is Professor or Dr. unless the person doesn't hold a Doctorate then you just go by Professor.
If it is staff with no distinction, then use Dear John/Jane Doe. No one cares about Mr. Mrs. or Ms. anymore. Maybe some baby boomers might still care, but even then.. also, I've seen people get very upset and offended by being addressed as Mrs. and they were not married. Best just to avoid this one if you ask me.