It is important to be careful when addressing a letter, especially if it is to someone you do not know personally. Although most people are not very formal in their approach, some do have preferences as to how they should be addressed.

 Informal letters do not pose a problem in terms of selecting the correct way to address.  In contrast, formal or business letters need to be given more thought. So to get the desired job done, follow a standard procedure. As a rule, always mention the date on the letter. Include your full name, designation, your company's name and address. Including a telephone number and email address will speed up the response. When sending a letter to an organization, try and find out who will actually read your letter. Check if you have any previous correspondence you've received from them to see who has sent it. Alternately, call the organization and confirm the name and designation of the person you need to send the letter to. Mentioning the name of the person will give a better impression. Using an opening such as ‘to whom it may concern' sounds careless. So avoid using phrases like these at all costs. A systematic way would be to first write the receiver's name, followed by their designation and the name of the organization they are working for. For instance: Ms Melanie Jones, The Secretary, ABC Insurance Company. If you are unable to find out information about the recipient, do not try and send it to the organization in general. Address it to the department that would be likely to deal with the letter. When greeting the person, use Mr. or Dear Mr. (last name) if the person is a male. If the person is a female, use Mrs.  or Miss. only if you are sure of their marital status. If you are unsure, use Ms. or Dear Ms. (last name). 

So, when addressing a letter, follow the basic rules. The matter should be precise and to the point, yet make sure you do not leave out important information. Making mistakes might result in unfavorable consequences. End the letter with a simple ‘Yours Sincerely' followed by your signature.