What to do - or: Am I important or not?

I am sometimes not sure how to react or if I should like it. Imagine I am sending an e-mail to people, which I consider important, and don't get any response or feedback back. Okay - I don't expect a mail back immediately, but if the days drag on - how to react? I understand that people don't respond back immediately, as they are also busy - not just me.

The following questions come to my mind:
1.) Am I important?
2.) Was my mail important to me but not to the recipient?
3.) Is the recipient still alive (well, it could happen, if it is somebody, with whom you don't have contact on a regular base).

But the first two points are still valid, especially when you are in a working environment.