5.4.1收件地址拒绝：连接关闭[（一串错误代码，包括一个单词：保护）]如果您将此消息群发到多个地址，您每一个没有到达的地址都将接收到一个这样在提醒。手打个人翻译，满意记得采纳~
if the recipient's email address is incorrect, the email may fail to reach its intended destination. when you send an email, the email server attempts to deliver it based on the recipient's address. if the address is invalid or doesn't exist, the server may generate a bounce-back ...
The recipient address rejected error can occur if the email you’re trying to send the message to is inactive or no longer registered. However, it can also occur if your email is getting caught up by the recipient’s spam filter. Other than that, here are some more reasons it may occur...
if the recipient's email address is incorrect, the email may fail to reach its intended destination. when you send an email, the email server attempts to deliver it based on the recipient's address. if the address is invalid or doesn't exist, the server may generate a bounce-back ...