                onRcptTo: function(address, session, callback) {
                    if (!/@valid.recipient/.test(address.address)) {
                        return callback(new Error('Only user@valid.recipient is allowed to receive mail'));
                    }
                    if (/[\x80-\uFFFF]/.test(address.address) && !session.smtpUtf8) {
                        return callback(new Error('Trying to use Unicode address without declaring SMTPUTF8 first'));
                    }
                    return callback(); // Accept the address
                },
The software constructs a string for a command to executed by a separate component
in another control sphere, but it does not properly delimit the
intended arguments, options, or switches within that command string.