Unverified50% confidenceFactExact time
In the standard SMTP process, when an email is sent to a non-existent address, the receiving server returns a 5xx error code (such as 550 User Unknown) and the sender receives an NDR bounce notification.
1
Sources
50%
Confidence
Long-term
Relevance
8/9/2026
First Seen
Sources
Noreply邮箱的隐私陷阱：无人回复不等于无人查看
hackernewshackernews8/8/2026
Related Claims
Unverified退信（Bounce）或不可投递报告（NDR）是SMTP协议中通知发送方投递失败的标准机制，由RFC 3464规范定义79% similarUnverifiedSMTP协议存在致命缺陷：任何人都可以在邮件的发件人字段填写任意地址70% similarUnverifiedRFC 5321是IETF制定的SMTP标准，规定邮箱地址local-part的解释权归属于接收方邮件服务器66% similarUnverified邮件转发中Received字段记录每一跳服务器信息，Return-Path字段存储退信地址，X-Forwarded-To等扩展头字段可能暴露真实收件人64% similar
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