Beginning a letter with the proper etiquette is crucial for making the right impression. The way you begin a letter can communicate what you expect from the exchange and what you think of the recipient---beginning the wrong way, then, can make you seem rude, expectant or foolish. By exhibiting the proper etiquette, your letter can make you seem professional, responsible and respectful.
Always Greet Reader
You should always use a greeting at the beginning of a letter or other formal communication. Without it, you demonstrate a lack of concern for the other person, making you seem self-centered and disrespectful. Think of it as you would in-person communication---when you meet with someone, you always greet them first. (Beginning with "Dear" and the person's name is an acceptable greeting.
If you are writing a professional letter, you should use the appropriate salutation. Essentially, you should address the recipient as he would expect to be addressed. For example, "Dear Dr. Michaels," or "Dear Ms. Alexander." If you are unsure if the woman prefers "Miss" or "Mrs.," use "Ms." If you are unsure if the person is a man or a woman---"Sam Watson," for example---use the full name.
Never write a letter to "To Whom It May Concern." This is impersonal, and it makes it seem as though you do not know anything about the person to whom you are writing. You may at least use a pronoun, "Dear Sir or Madam," or a job title, "Dear Hiring Manager."
For informal questions and general inquiries, you may be less formal in your greeting. For example, you may write, "Hi, I was wondering if you had any ideas for dinner tonight." For less formal business communications, you can use the person's first name. For example, if sending an e-mail to someone else in your department, you may start with, "Hi Jeremy, I just wanted to make sure you got my sticky note."
When writing to two people, include both names. For example, you would write, "Dear Mr. Smith and Mrs. Jones" (Refrain from using general or informal language, like "Hey guys."