greeting people

When I got back after my first visit of the US, people though I was way too polite - I would smile a lot and say "hi" even to clerks in the bank or somewhere else (and we do not greet those we do not know). Partly based on this politeness I became very good friends with a girl who got back from UK. She became more polite too. We were known as the most polite people in our department.
Greeting is a norm as well as "how are you" question. Although people usually do not expect any negative answer here, and I'm totally fine with any answer as long if it's true. If greeting is a norm, does that mean that I'm in trouble when someone does not say "Hi" at the beginning of the letter to me?
Personally, I try to put "hi" or some kind of a greeting in any message, unless it is not one out of several messages in conversation. And when I do not get greeting from a person I feel that maybe I've done something bad.

Or is that a form of a conversation where greeting can be omitted? What about SMS? is it polite not to say "Hi" there?