If I am writing an email to a person I have never met and who is outside of my office, I will use "Dear Xxxxxxxx, ...". Also, to my grandmother. :) Using "hey" or "hi" is generally too informal and are considered unprofessional. To anyone within my office, I will write their name without dear/hey/hi. Just, "Xxxxxx, ..." To my friends, I will write "Hey/Hi Xxxxx, ...". I'm 26 years old, so perhaps those who are older will find my style too informal, but I think it is fairly normalized in the workplace now.

I agree, hey or hi is more informal and is not common in workplace. I use "dear" as the standard greeting for all professional emails. If I do not know the person or I am sending to a generic email address, I say "to whom it may concern,"

I agree with Patrick! Use 'Dear Sir/Madam/John' unless you know the person well or it is a personal message.