Addressing a letter has more to it than writing a name and a mailing address. Or just writing a name at the top of a note with a comma behind it. Many subtleties and formalities should be taken into account. They are based primarily on the letter writer's relationship with whoever is receiving the letter. There is also the aspect of where the letter is going and whether it should be addressed to a particular person, a business or a group of people. Below are the rules of thumb for properly addressing letters.
First, who is this letter being written to? Is it a close friend? If so, you can write just about anything, including a cutesy nickname or their initials. This conveys a little extra connection to the person who is receiving the letter in the first place. However, if it's someone you don't know well or at all, addressing a message formally is paramount. Use the titles of Dr. / Mr. / Mrs. or Ms. before their name and maybe even a professional title of some kind after.
Next, keep in mind the occasion. Is this a letter from a foreign place you have visited that is just for fun? Or is this a letter telling someone they have a job or don't? All of these factors play into the format of a letter. Especially how it addressed to the person, who is receiving it. While much of this will play out in the body of the letter itself, it should all start with the address.
Addresses in business letters take on a whole new aspect. Certain formalities must be taken into account to ensure you are taken seriously from the start by an employer, employee, client or business associate. These letters must be addressed in the following way: the sender's address and contact information, then the date, then the inside address and the salutation. Breaking from this process will often reflect poorly on the sender and make them less professional before the content of the letter is ever even read.
Titles are essential when formally addressing someone in a letter; it's kind of critical to get those titles right. You don't want to address a recently married woman as Ms. or not call someone a doctor Dr. in the letter. It will lead to instant offense or a thought that you don't do your research or care. Do what you can to ensure you get this correct in your letter.
When you are sending a letter to two people who happen to be a couple and living together, be sure to address them correctly. If they aren't married, then address them each by their full names and use Ms. for the woman. Perhaps they are married but don't share the same last name? Then address them both by their full names and use Mrs. for the woman. If they are married and share the same last name, use Mr. and Mrs. and just the last name.
When sending a letter to a whole family, the best way to do it is to address the entire family at once. Even if they don't all share the same last name. Going by The Smiths, for example, is better than writing out each family member's name individually.
Many times, letters will have to be addressed to no one in particular as per the instructions given by a business or organization. So if you're sending the letter to Customer Service or the Hiring Manager, you won't be able to address your letter to a single human being. The best way to go in this case is by using To Whom It May Concern or Dear Customer Service, or the name of the informal group you are writing the letter to in the first place.
If you're typing your letter, which is what most people do these days, the font is critical for first impressions and impression throughout the subsequent letter itself. There are many acceptable fonts out there. Each one conveys a slightly different feeling. Try to stay away from fonts that are cartoonish, as you won't be taken seriously, or hard to read, as your letter will be difficult to decipher.
Providing your contact information on the letter itself in the address area is important, as long as it is necessary. If you are writing a business letter and want or expect some response, include your important details. If you are writing a formal, personal letter to someone, include the easiest way to get in touch with you, so you are sure you receive any correspondence back immediately and directly.
For letters you are mailing, the envelope will be the first impression. The main address for the person you are sending it to should mirror the address on the letter inside. If you have included it there. It's always a good idea to include a return address in the upper left-hand corner of the envelope, so the letter can come back if it can't be delivered. You only have to put an address here as well, rather than a name, as it will help keep anonymity throughout the mailing process.