RFC1894 defines a set of MIME content types for relaying information about the status of electronic mail messages. These can be thought of as ``return receipts.'' Berkeley sendmail 8.8.x currently has some command line options in which the mail client can make requests as to what type of status messages should be returned.
To support this, there are two variables. $dsn_notify is used to request receipts for different results (such as failed message, message delivered, etc.). $dsn_return requests how much of your message should be returned with the receipt (headers or full message). Refer to the man page on sendmail for more details on DSN.