How should staff address non-doctorate professors? Background:

I am in an administrative staff position, in a research compliance role, and frequently have to "cold" email professors I have never met or otherwise previously interacted with.  In my institution almost all professors have MD degrees, so I typically address them as "Dr. X."  However there are a small number of professors with master's degrees in areas of specialty that don't offer doctorate-level degrees, and I am uncertain how to appropriately address these individuals.

My gut reaction is to address these individuals by their first name (as our institution's culture considers first name acceptable for staff above you in the reporting chain), but to me this feels disrespectful when applied to faculty, especially considering the content of my messages are often directing them to do things (or stop doing things) they'd rather not.  Conversely, calling them "professor" seems unusual to me since I am not enrolled at the institution.  Finally, my school is very progressive, and I worry about (mis-)using gendered pronouns.

Question:

Am I worried about this unnecessarily?  How should I address these professors when I can't open by asking how they would like to be addressed?
as our institution's culture considers first name acceptable for staff above you in the reporting chain


First name it is then. Or you could go first and last name without a title. Or you can drop the name completely and just get on with telling them what they have to do. It sounds like you know they are all professors/faculty, which would mean using professor is not incorrect, although it is more formal than just the first name and the same as calling someone doctor.

I would steer away from Mr/Ms/Mrs for a couple of reasons. First, it introduces the gender issue you raise. Second, Professor is a higher honorific than Ms/Mr. Finally, working in a medical field where half the clinicians have a terminal masters and half have a terminal doctorate, I find it mildly insulting to use different titles for two people who have terminal degrees.