Help regarding salutation in letter writing! (see details)

Recently, I am sending and replying emails (and sometimes mails) to people in US. Letters I am sending are official and formal. I need your advice and opinions about salutation or greetings that go at the beginning of the letters. Should I use their names or just sir or madam will do?

Generally, I start with Dear Sir or Dear Madam or Dear Sir/Madam when I am contacting them for the first time. I do this also when I haven’t personally met them. After meeting them personally and feeling that they are comfortable with me, I use their names (example: Dear Justin).

Interestingly, when I get (email) replies, they don’t use any salutation or anything. They just start off with the mail body directly. Can I also reply without any salutation? Or is it always better for me to use it?

Kindly let me know what is the right way to do it. What would you have preferred? You can post answer even if you are not from US. Any other advice that relates to writing letters or emails is also appreciated.