Do you ever get spam emails where they don't know your name so they just say "Hey"?
As you can see in the GIF, this can come across as rude IF you don't add a first name.
So it might be OK to say "Hey Fiona" if you know the person (it really depends on the person, so be careful) but probably safer to use "Hi Fiona" or "Dear Fiona" if you're writing for the first time.
It's REALLY rude to have no form of address at all. Other things to avoid: