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I try not to overthink this stuff and just act like a human, but I get the sense that some recruiters are very particular.

My rule of thumb is to use the salutation/last name until I get an email signed with just the first name (or explicit permission/request to use first name). Has always worked.

Dear FIRSTNAME LASTNAME

I use the above as a sort of middle ground between the informality of addressing someone by their first name, and not wanting to err in misusing Ms./Mr. in the (rare) chance that someone identifies a particular way that isn't what I would assume.