opening/ closing/ greetings/ in an business mail

Generally, you want to know and use their name. You would start by saying "Dear Mr. Smith" for example. Then, go on and introduce your product in the first paragraph, what it does and how it would benefit your client. Finally, thank the person for their time and put either Sincerely, or Thank You at the bottom and your name about two to four spaces below that. I hope that helps.

I always use dear customer, thanks in advance or kind regards, name here For an email this is formal enough...

no

In writing a business letter, you need to be as official as possible. Try this link for essential details on how to write a business mail: http://esl.about.com/cs/onthejobenglish/a/a_basbletter.htm