I follow the robustness principle for emails: "Be conservative in what you send, be liberal in what you accept" :-)

Here's how what I've learned from [Postel's Law](http://en.wikipedia.org/wiki/Robustness_principle) applies in this scenario:

  - You cannot control what others send to you; thus, for maximum benefit you should accept everything.
  - You cannot control what others accept from you; thus, for maximum benefit you should send it in a way that everyone accepts.

Sure, you can be angry about formless emails and attempt to teach people, but this doesn't accomplish anything.

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 add-on 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 :-)