Letters consist of six parts: address, address, yearning, text, wish and signature. In letters, honorifics address each other to show respect. You can use the ancient titles, such as Jun and Gong, and you can also add the word Jing or the name before the title. For some venerable elders with certain academic status, the word "madam" is used exclusively to address the other party or other people's wives, and you must never address your wife as my wife.
Call the other person's son your son, also called your son's interest, which means your son's interest. Call each other a good brother, a good friend, a good friend, and a good friend. Generally speaking, calling each other's font size is more respectful than calling them names.