四种最常见的退信原因以及解决办法常见退信提示一：450 4.1.8: Sender address rejected: Domain not found解决办法：对方的域名解析有误，让对方检查一下自己邮箱的域名解析常见退信提示二：550 5.1.1: Recipient address rejected: User unknown in local recipient table解决办法：收件人地址不存在，粗心的你快检查收件