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 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 :-)