[退信分析] 收件人域名不存在:Host or domain name not found
[退信分析] SPF记录缺失:SPF check failed
[退信分析] IP灰名单:Blocklist/spamhaus
[退信分析] 收件方拒收:user reject
[退信分析] DNS反向解析错误:553 can't verify FROM domain in DNS
[退信分析] 垃圾邮件退信:Content reject/Spam denied
[退信分析] 发信数超限制:sending quota exceeded
[退信分析] 网络异常:rcpt handle timeout
[退信分析] 邮件过大:exceeded mailsize limit
[退信分析] 域名DNS异常:DNS query error
[退信分析] 收件人不存在:user not exist
[退信分析] 收件容量满:Quota exceeded