A greeting is just as important in an email. Not only does it say to whom the message is directed, it is also an acknowledgement of the person by name. To omit a greeting of any sort, or to state just the person's name before jumping into conversation may come across as abrupt and curt. If you browse through your emails of the past month or so, you will probably see a variety of salutations -- so which is appropriate to use when?

With email, it is always better to be more formal than too casual.  

• "Dear_____" is still used, although with much less frequency, and may be a good option when using an honorific, such as Mr., Ms., Dr. or Professor.  
• "Hello______" is also appropriate as it establishes a friendly, yet still respectful tone.  
• "Hi__________" is a more informal greeting that can be used once a relationship is established.

Group emails may be addressed in several different ways: "Greetings", "Hello Everyone / All", "Greetings, Team", "Colleagues", etc. If it is a small group, it is acceptable to address everyone by name. 

"Greetings", "Good Morning / Afternoon" or "Good Day" are also appropriate salutations; the time noted on the message will tell the person when you wrote and sent the email, so there is no need to be concerned about when it may be read (however, accuracy in your greeting is important as a discrepancy between your choice of greeting and the time stamp will be noticed.)

All sorts of information is available on the internet, so there is no excuse for sending an email addressed to "Dear Sir or Madam" or "To Whom It May Concern". Doing so says you cannot be bothered to put forth any effort to obtain the information and shows a lack of interest and sincerity. If for some reason you cannot find the correct names on-line, pick up the telephone and call. You are going to make an impression here, so you may as well make one that counts.