The word "dear" is a perfectly acceptable salutation in business communications and professional correspondence. It works whether you know the recipient's name or not. "Dear Writer" and "Dear Reader" are as appropriate as "Dear Mr. Adams" or "Dear Dr. and Mrs. Jones." If you know the recipient well, you might even write "Dear John."
The salutation "dear" is not necessary in simplified business correspondence. Particularly in the case of daily emails, it is appropriate to use just a person's name without the word "dear," as in: "Jeff, I was talking to..."
Hi Versus Hello
You can use "Hi" as a business salutation, but it is best reserved for people that you know personally. "Hello" is more formal for business associates and acquaintances.
Good Morning, Good Afternoon, Good Evening
Wishing someone well is a highly appropriate salutation in all cases, but it is best reserved for emails or correspondence that is received immediately. The time of day determines whether you wish someone "Good morning," "Good afternoon" or "Good evening." Morning hours are usually anytime between midnight and noon, afternoon hours are between noon and 5 pm and evening hours are usually from 5 pm up to midnight. "Good day" works at any hour, particularly if an email or letter is pre-written and scheduled for later delivery.