Summarize:

You may have letterhead that includes your name and address in a banner that prints along the top of the letter. If not, you should type your full address in either the top-left or top-right corner of the page. Each of the lines should be left-justified. Include any additional information, such as your job title, that is relevant to the subject of the letter or will help the recipient more easily identify you. After your address and the date line, double-space and begin the address of the recipient by listing their full name and any legal or courtesy title, such as "Mr.," "Ms.," or "Dr." For medical doctors, it is appropriate to use either "Dr." before the name, or "M.D." after their name, but don't use both. For example: "Dr. Jan White" or "Jan White, M.D." Leave off gendered courtesy titles such as "Mr." or "Ms." if the recipient has a gender-neutral name, or if you're unsure about their gender identity. Use the job title that correctly corresponds to the purpose of your letter. If the recipient's job title is irrelevant to your letter, or you're writing to them personally, you may choose to leave the job title out. Including the job title means you're addressing the recipient in that capacity. For example, if someone is both President and CEO of a company, you may address them as President, as CEO, or as both. Following the recipient's name and any job titles, move down to the next line and type the full legal name of the business. Check the website or business publications to make sure you get the exact name. Words such as "corporation" should only be abbreviated if they are done so in the legal name of the business. Capitalize and space words exactly as they are done in the business's legal name, including any special characters. For example, if the recipient works at Yahoo, write the business name on your letter address as "Yahoo!", which is the registered legal name of the business. On the next lines, provide the street address, suite or office number, and other address information completely written out with no abbreviations and correct punctuation. Use numerals for street numbers, unless the street number is part of the name of the building, such as "One Town Plaza." Spell out the full name of the city, then place a comma, then type out the full name of the state or province and the postal code. If it's an international letter, include the full name of the country on the last line. While the address itself typically should be single-spaced, you'll want to include a blank line before writing the salutation. Then double-space again before the body of the letter.
Place your address at the top of your letter. List the name of the recipient with the appropriate courtesy title. Provide the recipient's job title. Include the name of the business. Write out the recipient's address. Double space before starting your letter.