I follow the robustness principle for emails: "Be conservative in what you send, be liberal in what you accept" :-)
So I always use a short envelope but don't mind if others don't, in fact I appreciate it as it saves time.
By the way I use the Quicktext addon for Thunderbird for handling the formalities for me. Hitting Alt+1 creates:
Hi [First Name],
[Cursor]
Regards, Me
Alt+2 then is the more formal version and so on. So you can write quickly and still be a bit nice :-)