Advice: Email communications should have a proper salutation and leaving it blank or using something like “Hey,” is inappropriate.
Why: Email communication to faculty should be viewed as professional communication. Leaving the salutation blank or starting off with something like “Hey” will come off as rude. It is ok to use you instructor’s first name such as Dear Tom or just Tom if your instructor has invited you to do so. If you are in doubt, then it is better to be too formal than inappropriately informal.