A mail stop is a delivery point where mail is delivered and collected at large facilities, such as a university campus, a government agency or a large business. It is generally denoted through the use of a mail stop code, which is typically alphanumeric.
While a mail stop code may be somewhat inscrutable to the outside observer, the mail delivery staff at the facility decodes the information, often deciphering information relating to the building, the floor and even the department or desk where the mail must be delivered.
Mail stop codes are used for both internal mail within the facility as well as mail delivered to the facility. When addressing mail to be delivered to a mail stop facility in the United States, state the recipient's name, the recipient's mail stop code, the name of the department, the name of the facility and then the street address, followed by the city, state and ZIP code.