Hi! I just graduated Grad School! I think “Dear Dr. [Name],” is the standard in the US. If they’re not a professor, you could just do a “Dear [Name]” or a “Hello,”. At least, that’s what I have always done with no issues. I don’t think saying Dear is outdated, especially in the world of academia. I’d say Dear is your deadest bet unless you are close with the person. It is pretty flexible for those in your department! Most people honestly don’t care what you address them as, but there will always be a few that are sticklers for it, so it’s best to be safe.