Is Vade's use of 550 5.7.1 a hard bounce, and what do the error codes mean?

Key findings

• Hard bounce classification: A 550 SMTP error code indicates a permanent failure, making the 550 5.7.1 bounce a hard bounce. This means the email cannot be delivered to that address.
• Vade's specific meaning: Vade Secure uses the internal code OXSUS001_403 in conjunction with 550 5.7.1 to specifically signify that the email was sent to a non-existent user.
• Standard code misalignment: According to IANA's SMTP enhanced status codes, 5.7.1 generally means delivery not authorized or message refused, while 5.1.1 is the standard code for a bad or non-existent mailbox address. This implies Vade's use of 5.7.1 for user unknown is non-standard.

Key considerations

• Bounce handling: Regardless of Vade's specific internal code, a 550 error indicates a hard bounce. Senders should treat these as permanent failures and immediately suppress the affected email addresses from their mailing lists to maintain good deliverability and avoid reputation damage. Learn more about hard and soft bounces.
• Interpreting error messages: While the primary SMTP code 550 is key, always examine the full bounce message, including any enhanced status codes (like 5.7.1) and accompanying text or URLs, for more specific diagnostic information. This can help classify and manage SMTP bounce codes more effectively.
• Vendor communication: If you are an ESP or a large sender, engaging with providers like Vade Secure directly through their postmaster or sender tools can sometimes clarify these non-standard error messages or even encourage them to align with RFC standards. Consulting the guide to email bounce codes can provide further context.

Key opinions

• Default to hard bounce: Many marketers automatically categorize 550 5.7.1 from Vade as a hard bounce, especially when Vade's own documentation implies a permanent issue like a non-existent user.
• Intent over exact code: Some marketers focus on the clear intent behind Vade's error message (e.g., _403 meaning non-existent user) rather than strictly adhering to the 5.7.1 enhanced status code, which has a different standard meaning.
• ESP blame game: There can be disagreements with Email Service Providers (ESPs) regarding the interpretation of such non-standard bounce codes, with ESPs sometimes blaming the receiving server (Vade) for using an incorrect code.

Key considerations

• List suppression: Treating a 550 error from Vade as a hard bounce and immediately suppressing the address is crucial for maintaining good sending reputation, preventing further bounces, and avoiding potential blocklisting. Read more on resending emails to hard bounces.
• Monitoring documentation: It is important for marketers to monitor and refer to the specific documentation provided by Vade or similar security platforms to understand their proprietary error codes, even if they deviate from standard RFC definitions.
• Engaging with postmasters: Utilizing postmaster tools or contacting the receiving domain's administrators (like Vade) can sometimes help clarify issues or provide avenues for remediation. Tools like those described in the automizy bounce code guide can be helpful.

Key opinions

• RFC compliance: Experts stress that SMTP error codes are governed by RFCs, meaning the first three digits (e.g., 550) are the primary determinant of the bounce type (hard vs. soft), not internal, non-standard codes or URLs.
• Misleading enhanced codes: Vade's use of 550 5.7.1 for user unknown is problematic because 5.7.1 has a different, well-defined meaning in the IANA enhanced status codes, specifically related to authorization or refusal. The correct code for an unknown user is 550 5.1.1.
• Complexity for senders: Experts lament that non-standard practices by mail receivers, such as using a single generic SMTP response for various types of failures, force senders to consult external web pages to decipher the actual reason for the bounce, increasing the complexity of bounce handling.

Key considerations

• Adherence to standards: Senders and ESPs should prioritize parsing the initial three-digit SMTP code as the definitive indicator of a bounce type. While additional information is helpful, the core classification should follow RFC standards for consistent bounce management and mailing list suppression.
• Advocate for clarity: The deliverability community should encourage mail filtering services and ISPs to use accurate and standard SMTP and enhanced status codes to reduce confusion and streamline bounce processing. This helps troubleshoot SMTP 550 errors.
• Comprehensive analysis: Despite inconsistencies, it's still beneficial to review the entire bounce message, including any proprietary codes or embedded links, as they can sometimes offer additional context, even if the primary SMTP code is sufficient for bounce classification. More insights can be found in email bounce error codes guides.

Key findings

• SMTP class 5xx: According to RFCs, any SMTP reply starting with 5xx indicates a permanent failure, classifying it as a hard bounce. This means no further attempts to send to that address will succeed.
• Enhanced status code 5.7.1: The IANA registry defines 5.7.1 as delivery not authorized, message refused. This implies general security, policy, or authentication issues, not an invalid recipient.
• Enhanced status code 5.1.1: The standard and correct enhanced status code for a bad destination mailbox address or user unknown is 5.1.1, according to IANA and RFCs.

Key considerations

• Reliance on official standards: When processing bounce messages, automated systems and human operators should primarily rely on the official RFC and IANA definitions for SMTP and enhanced status codes, as these are universally understood and intended for machine-readable interpretation.
• Mitigating non-standard practices: While some mail receivers use non-standard codes, the overarching 5xx prefix should still guide the determination of a hard bounce and subsequent suppression action. This helps mitigate the impact of potentially confusing, non-compliant bounce messages.
• Promoting consistency: There is a continuous need for mail servers and security platforms to adopt consistent and RFC-compliant error reporting to foster a more reliable and predictable email ecosystem. This ensures that enhanced status codes are used as intended.