#10: Good, maybe my comment about business correspondence was too generalized. I was simply drawing on my own experience and , of course, that's not applicable for everyone.
But when I worked as a secretary, it would have been unthinkable to write anything other than Dear Sirs, or Dear Mr A, Dear Mrs/Ms B etc. I did not know any of my correspondents personally. And I still address my customers with "Hello" or "Good morning" etc, even if I do know them personally. I only ever use "Hi" for friends. While "Hey!" may now be considered friendly, it certainly wasn't always that way. It's probably also a generational thing and I'm simply showing my age!
The trend is towards being more casual and that's not necessarily a bad thing. But I still hate being addressed with "Hey", especially in English, probably because it was so deeply ingrained in me when I was growing up that it was derogatory and impolite.
In German, I don't find it quite as negative. I stand by my conviction that "Hey you!" is impolite.
To return to the OP, I'd say Salutations could never sound right to me at the beginning of an e-mail, however informal, but could possibly be used (tongue-in-cheek) at the end.