may be used to report that value. Note that beside the numerical 3-digit value, the SMTP response contains itself a human readable part. The information Remote-MTA: dns; smtp.store.example [192.0.2.3] Diagnostic-Code: smtp; 550 No such user here is sometimes reported as, e.g., while talking to smtp.store.example [192.0.2.3] >>> RCPT TO:<nonexistinguser@store.example> <<< 550 No such user here Security implications On 10/04/2021 an Australian Security Researcher (Sebastian Salla) demonstrated how email bounce messages could be abused to reduce the operational effectiveness of email spam and malware filters. The default behaviour for many email gateways is to include an untampered copy of inbound message headers within bounce messages. Through analysis of these message headers, a threat actor is able to positively identify whether or not their email would end up in a targets mailbox. This attack can be scaled to encompass dozens of vulnerable targets which improves the efficiency of phishing campaigns. See also Backscatter (Backscatter of email spam) Bounce Address Tag Validation (BATV) Email tracking Sender Policy Framework (SPF) DomainKeys Identified Mail (DKIM) Sender Rewriting Scheme (SRS) Simple Mail Transfer Protocol (SMTP) Variable envelope return path (VERP) Related RFCs - Simple Mail Transfer Protocol - Simple Mail Transfer Protocol (SMTP) Service Extension for Delivery Status Notifications (DSNs) - The Multipart/Report Media Type for the Reporting of Mail System Administrative Messages - Enhanced Status Codes for SMTP - An Extensible Message Format for Delivery Status Notifications - Recommendations for Automatic Responses to Electronic Mail - Internationalized Delivery Status and Disposition Notifications References External links Mail DDoS Attacks through Non Delivery Messages Microsoft DSNs and NDRs in Exchange Server Understanding Bounce Email Email Email authentication Internet Standards