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 :-)