According to USPS.com, international addresses need the recipient's name, the street name with building and apartment number, the name of the neighborhood or village, the postal code or delivery zone number with the city and state, and the destination country. Each unit of information should be on a separate line.
An example of a good address for the fictitious Mr. Juan Gomez of 900 Mercado St apartment 2 in the Xochitl neighborhood which lies in postal code 77520 of the city of Cancun, province of Quintana Roo, country of Mexico:
SR. JUAN GOMEZ
MERCADO 900 - 2
77520 CANCUN, Q. ROO
USPS further advises using all capital letters. Reserve the entire right side of the envelope for the address. Maintain a border of five-eighths of an inch around the edge of the envelope for digital stickers.