python 使用 smtp 无法向多人发邮件？

给单个人地址发就没问题，多人 list 就不行了。

这是报错内容

Error in functon : " send_text_email " ,
     Error name is : " SMTPDataError " ,
     Error type is : " <class 'smtplib.SMTPDataError'> " ,
     Error Message is : " (554, 'DT:SPM 126 smtp2,DMmowECZeEbGLkNX_iNcAA--.205S2 1464020678,please see http://mail.163.com/help/help_spam_16.htm?ip=123.150.243.7&hostid=smtp2&time=1464020678') " ,
     Error doc is : " The SMTP server didn't accept the data. " , 

msg 内容

From nobody Tue May 24 00:24:38 2016
Content-Type: text/plain; charset="utf-8"
MIME-Version: 1.0
Content-Transfer-Encoding: base64
From: =?utf-8?b?5rWL6K+V5py65Zmo5Lq6?= <[email protected]>
Subject: test
To: [email protected],[email protected]

aW5mbw==

主要代码

def send_text_email(self, title, message, message_type):
        """
        发送文本邮件
        没有返回值
        函数内调用_format_address()
        :param message_type:
        :param message:
        :param title:
        """
        print 'send_text_email %s ' % title
        msg = MIMEText(message, 'plain', 'utf-8') 
        msg['From'] = self._format_address(u'%s<%s>' % (self.mailName, self.sender))
        msg['Subject'] = Header(title)
        while 1:
            try:
                smtp = smtplib.SMTP()
                # smtp.set_debuglevel(1)
                print '开始尝试连接邮箱'
                smtp.connect(self.smtp_server, self.smtp_server_port)
                print '成功连接邮箱'
                print '开始尝试登陆邮箱'
                smtp.login(self.username, self.password)
                print '成功登陆邮箱'
                if message_type == "securityInfo":
                    print '开始发送事件邮件'
                    msg['To'] = ','.join(self.receiver)
                    print 'msg', msg
                    smtp.sendmail(self.sender, self.receiver, msg.as_string())
                    print '成功发送事件邮件'
            except smtplib.SMTPAuthenticationError:
                print '认证失败,邮箱连接可能出问题了'
                self.count += 1
                if self.count < 3:
                    time.sleep(10)
                    continue
                else:
                    print '更换邮箱后重试...'
                    self.Mail_choose = not self.Mail_choose
                    self.mail_init()
                    self.count = 0
                    continue
            except Exception as e:
                error_text = exception_format(get_current_function_name(), e)
                print error_text
                time.sleep(10)
                continue
            else:
                smtp.quit()
                self.count = 0
                break

求各位指教