邮件客户端发信失败

邮件客户端发信失败相关问题共 8 问：

  1. 发邮件被退信"550 5.7.1 messages from [IP] weren't sent"，没有列在黑名单，怎么解决？
  2. SMTP 发信报错"550 5.7.1 Unable to relay for xxx@xxx.com"或"Relaying denied"，怎么解决？
  3. 发给 Outlook.com/Hotmail 退信"550 5.7.708 Access denied, traffic not accepted from this IP"，没有在黑名单上，怎么办？
  4. 发给 Gmail 退信"552 5.7.0 This message was blocked because its content presents a potential security issue"，附件被 Q拒绝怎么办？哪些文件类型被永久封禁？
  5. 邮件客户端发信报"535 5.7.8 Authentication failed"，密码明明没错？
  6. 退信提示"550 5.7.1 Unable to relay"，发不出邮件怎么修？
  7. 邮件客户端提示"无法连接到 SMTP 发信服务器"（连接超时或拒绝），怎么排查？
  8. 邮件客户端发件箱里的邮件一直发不出去，卡在那里，怎么排错？