Write out your full name including your formal title (e.g., Mr. John Doe) approximately 1/8 to 1/4 inch from the left and top edges of the envelope. Replace your name with the full title of your company as registered with the Secretary of State and licensing agencies (e.g., The Best Envelope Company) if you are addressing based on your business.
Keep in mind that wives are addressed based on their husband's name (e.g., Mrs. John Doe) and that you only use the husband's name once if you are addressing a couple (e.g., Mr. and Mrs. John Doe). If one recipient has a different last name or formal title, write out both names (e.g., Mr. John Doe and Dr. Jane Williams).
Write out the post office box number (e.g., P.O. Box 123) directly below your name or company name. Left justify this line to your name line. Omit this step if you don't have a P.O. box.
Write out your street address (e.g., 1234 Some Street) directly below the P.O. box line (or name line, if there is no P.O. box). Left justify this line to your name line (and P.O. box line, if applicable). Do NOT abbreviate, as a truly formal envelope head does not use punctuation, with the exception of titles.
Write your city, followed by a comma and a space, underneath all previous lines, left justified. Write your full state name on the same line (again, etiquette says not to abbreviate, even though the post office prefers states to be abbreviated), followed by a space and your 5-digit zip code.
Find the center of the envelope. Write out your recipient's formal name or company name.
Write out your recipient's P.O. Box left justified directly beneath his name or company name. Omit this step if there is no P.O. box.
Write your recipient's street address, left justified underneath her name and P.O. box lines (if applicable).
Write your recipient's city, followed by a comma and a space, underneath all previous recipient lines, left justified. Finish this line by writing out your recipient's full state name, followed by a space and his 5-digit zip code.
Place the envelope stamp in the top right corner, about 1/8 to 1/4 inch from the top and right edges.